71V016HSA15PH8 - INTEGRATED DEVICE TECHNOLOGY

FUNCTIONAL BLOCK DIAGRAM

BUF

VDD

CLKIN

AD7740

VIN

FOUT

2.5V

REFERENCE

VOLTAGE-TO-

FREQUENCY

MODULATOR

CLKOUT

GND

CLOCK

GENERATION

REFIN/OUT

REV. A

Information furnished by Analog Devices is believed to be accurate and

reliable. However, no responsibility is assumed by Analog Devices for its

use, nor for any infringements of patents or other rights of third parties

which may result from its use. No license is granted by implication or

otherwise under any patent or patent rights of Analog Devices.

AD7740*

FEATURES

Synchronous Operation

Full-Scale Frequency Set by External System Clock

8-Lead SOT-23 and 8-Lead microSOIC Packages

3 V or 5 V Operation

Low Power: 3 mW (Typ)

Nominal Input Range: 0 to VREF

True –150 mV Capability Without Charge Pump

VREF Range: 2.5 V to VDD

Internal 2.5 V Reference

1 MHz Max Input Frequency

Selectable High Impedance Buffered Input

Minimal External Components Required

APPLICATIONS

Isolation of High Common-Mode Voltages

Low-Cost Analog-to-Digital Conversion

Battery Monitoring

Automotive Sensing

GENERAL DESCRIPTION

The AD7740 is a low-cost, ultrasmall synchronous Voltage-to-

Frequency Converter (VFC). It works from a single 3.0 V to

3.6 V or 4.75 V to 5.25 V supply consuming 0.9 mA. The AD7740

is available in an 8-lead SOT-23 and also in an 8-lead microSOIC

package. Small package, low cost and ease of use were major

design goals for this product. The part contains an on-chip 2.5 V

bandgap reference but the user may overdrive this using an

external reference. This external reference range includes VDD.

The full-scale output frequency is synchronous with the clock

signal on the CLKIN pin. This clock can be generated with the

addition of an external crystal (or resonator) or supplied from a

CMOS-compatible clock source. The part has a maximum

input frequency of 1 MHz.

For an analog input signal that goes from 0 V to V

REF

, the out-

put frequency goes from 10% to 90% of f

CLKIN.

In buffered mode,

the part provides a very high input impedance and accepts a

range of 0.1 V to VDD – 0.2 V on the VIN pin. There is also

an unbuffered mode of operation that allows VIN to go from

–0.15 V to VDD + 0.15 V. The modes are interchangeable using

the BUF pin.

The AD7740 (Y Grade) is guaranteed over the automotive

temperature range of –40°C to +105°C. The AD7740 (K Grade)

is guaranteed from 0°C to 85°C.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 781/329-4700 World Wide Web Site: http://www.analog.com

Fax: 781/326-8703 © Analog Devices, Inc., 2001

PRODUCT HIGHLIGHTS

1. The AD7740 is a single channel, single-ended VFC. It is

available in 8-lead SOT-23 and 8-lead microSOIC packages,

and is intended for low-cost applications. The AD7740 offers

considerable space saving over alternative solutions.

2. The AD7740 operates from a single 3.0 V to 3.6 V or 4.75 V

to 5.25 V supply and consumes typically 0.9 mA when the

input is unbuffered. It also contains an automatic power-down

function.

3. The AD7740 does not require external resistors and capaci-

tors to set the output frequency. The maximum output

frequency is set by a crystal or a clock. No trimming or cali-

bration is required.

4. The analog input can be taken to 150 mV below GND for

true bipolar operation.

5. The speciﬁed voltage reference range on REFIN is from

2.5 V to the supply voltage, VDD.

3 V/5 V Low Power, Synchronous

Voltage-to-Frequency Converter

*Protected under U.S. Patent # 6,147,528.

AD7740 SPECIFICATIONS

K, Y Versions

Parameter

Min Typ Max Unit Test Conditions/Comments

DC PERFORMANCE

Integral Nonlinearity

CLKIN = 32 kHz

±0.012 % of Span

Unbuffered Mode, External Clock at CLKIN

CLKIN = 1 MHz ±0.012 % of Span Unbuffered Mode, Crystal at CLKIN

CLKIN = 32 kHz

±0.018 % of Span Buffered Mode, External Clock at CLKIN

CLKIN = 1 MHz ±0.018 % of Span Buffered Mode, Crystal at CLKIN

Offset Error ±7±35 mV Unbuffered Mode, VIN = 0 V

±7±35 mV Buffered Mode, VIN = 0.1 V

Gain Error ±0.1 ±0.7 % of Span

Offset Error Drift

±20 µV/°C

Gain Error Drift

±4 ppm of Span/°C

Power Supply Rejection Ratio

–55 dB ∆VDD = ±5% (5 V)

–65 dB ∆VDD = ±10% (3.3 V)

ANALOG INPUT, VIN

Nominal Input Span 0 – V

REF

V±150 mV Overrange Available

0.1 VDD – 0.2 V Buffered Mode

Input Current 8 10 µA Unbuffered Mode, VIN = 5.4 V, REFIN = 5.25 V

5 100 nA Buffered Mode, VIN = 0.1 V, REFIN = 2.5 V

REFERENCE VOLTAGE

REFIN

Nominal Input Voltage 2.5 VDD V

REFOUT

Output Voltage 2.3 2.5 2.7 V

Output Impedance

1kΩSee Pin Function Description

Reference Drift

±50 ppm/°C

Line Rejection

–75 dB ∆VDD = ±5% (5 V)

Line Rejection

–60 dB ∆VDD = ±10% (3.3 V)

Reference Noise (0.1 Hz to 10 Hz)

100 µV p–p

FOUT OUTPUT

Nominal Frequency Span 0.1 f

CLKIN

to 0.9 f

CLKIN

Hz VIN = 0 V to V

REF

. See Figure 2

LOGIC INPUTS (CLKIN, BUF)

CLKIN

Input Frequency 32 1000 kHz For Speciﬁed Performance

Input High Voltage, V

3.5 V VDD = 5 V ± 5%

Input High Voltage, V

2.5 V VDD = 3.3 V ± 10%

Input Low Voltage, V

0.8 V VDD = 5 V ± 5%

Input Low Voltage, V

0.4 V VDD = 3.3 V ± 10%

Input Current ±2µA VIN = 0 V to V

Pin Capacitance 3 10 pF

BUF

Input High Voltage, V

2.4 V VDD = 5 V ± 5%

Input High Voltage, V

2.1 V VDD = 3.3 V ± 10%

Input Low Voltage, V

0.8 V VDD = 5 V ± 5%

Input Low Voltage, V

0.4 V VDD = 3.3 V ± 10%

Input Current ±100 nA

Pin Capacitance 3 10 pF

LOGIC OUTPUTS (FOUT, CLKOUT)

Output High Voltage, V

4.0 V Output Sourcing 200 µA

. VDD = 5 V ± 5%

Output High Voltage, V

2.1 V Output Sourcing 200 µA

. VDD = 3.3 V ± 10%

Output Low Voltage, V

0.1 0.4 V Output Sinking 1.6 mA

POWER REQUIREMENTS

DD7

3.0 5.25 V

(Normal Mode)

0.9 1.25 mA V

= VDD, V

= GND. Unbuffered Mode

(Normal Mode)

1.1 1.5 mA V

= VDD, V

= GND. Buffered Mode

(Power-Down) 30 100 µA

Power-Up Time

30 µs Exiting Power-Down (Ext. Clock at CLKIN)

NOTES

Temperature range: K Version, 0°C to +85°C; Y Version, –40°C to +105°C; typical speciﬁcations are at 25°C.

See Terminology.

Guaranteed by design and characterization, not production tested.

Span = Max output frequency–Min output frequency.

Because this pin is bidirectional, any external reference must be capable of sinking/sourcing 400 µA in order to overdrive the internal reference.

These logic levels apply to CLKOUT only when it is loaded with one CMOS load.

Operation at VDD = 2.7 V is also possible with degraded specifications.

Outputs unloaded. I

increases by C

× V

OUT

× f

FOUT

when FOUT is loaded. If using a crystal/resonator as the clock source, I

will vary depending on the crystal/resonator

type (see Clock Generation section).

Speciﬁcations subject to change without notice.

REV. A

–2–

(VDD = 3.0 V to 3.6 V, 4.75 V to 5.25 V, GND = 0 V, REFIN = 2.5 V; CLKIN = 1 MHz; All

specifications TMIN to TMAX unless otherwise noted.)

AD7740

REV. A –3–

(VDD = 3.0 V to 3.6 V, 4.75 V to 5.25 V, GND = O V, REFIN = 2.5 V)

Limit at T

MIN

, T

MAX

Limit at T

MIN

, T

MAX

Parameter VDD = 3.0 V to 3.6 V VDD = 4.75 V to 5.25 V Unit Conditions/Comments

CLKIN

32 32 kHz min Clock Frequency

1 1 MHz max

HIGH

LOW

40:60 40:60 min Clock Mark/Space Ratio

60:40 60:40 max

50 35 ns typ CLKIN Edge to FOUT Edge Delay

2.3 1.8 ns typ FOUT Rise Time

1.6 1.4 ns typ FOUT Fall Time

HIGH

±20

HIGH

±8

ns typ FOUT Pulsewidth

NOTES

Guaranteed by design and characterization, not production tested.

All input signals are speciﬁed with tr = tf = 5 ns (10% to 90% of VDD) and timed from a voltage level of (V

+ V

)/2.

See Figure 1.

Speciﬁcations subject to change without notice.

tHIGH

tLOW

CLKIN

FOUT

Figure 1. Timing Diagram

TIMING CHARACTERISTICS

1, 2, 3

ABSOLUTE MAXIMUM RATINGS*

= 25°C unless otherwise noted)

VDD to GND . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V

Analog Input Voltage to GND . . . . . . . . –0.3 V to V

+ 0.3 V

Reference Input Voltage to GND . . . . . –0.3 V to V

+ 0.3 V

Logic Input Voltage to GND . . . . . . . . –0.3 V to VDD + 0.3 V

FOUT Voltage to GND . . . . . . . . . . . –0.3 V to VDD + 0.3 V

Operating Temperature Range

Commercial (K Version) . . . . . . . . . . . . . . . . 0°C to +85°C

Automotive (Y Version) . . . . . . . . . . . . . . –40°C to +105°C

Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C

Junction Temperature (T

Max) . . . . . . . . . . . . . . . . . . 150°C

SOT-23 Package

Power Dissipation . . . . . . . . . . . . . . . . . . (T

Max – T

)/θ

Thermal Impedance . . . . . . . . . . . . . . . . . . . . . 240°C/W

Lead Temperature (10 secs) . . . . . . . . . . . . . . . . . . 300°C

Reflow Soldering

Peak Temperature . . . . . . . . . . . . . . . . . . . . 220 + 5/0°C

Time at Peak Temperature . . . . . . . . . . . . 10 sec to 40 sec

microSOIC Package

Power Dissipation . . . . . . . . . . . . . . . . . (T

Max – T

)/θ

Thermal Impedance . . . . . . . . . . . . . . . . . . . . . 206°C/W

Thermal Impedance . . . . . . . . . . . . . . . . . . . . . . 44°C/W

Lead Temperature (10 secs) . . . . . . . . . . . . . . . . . . . 300°C

Reflow Soldering

Peak Temperature . . . . . . . . . . . . . . . . . . . . . . 220 +5/0°C

Time at Peak Temperature . . . . . . . . . . . . . 10 sec to 40 sec

*Stresses above those listed under Absolute Maximum Ratings may cause perma-

nent damage to the device. This is a stress rating only; functional operation of the

device at these or any other conditions above those listed in the operational

sections of this speciﬁcation is not implied. Exposure to absolute maximum rating

conditions for extended periods may affect device reliability.

CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily

accumulate on the human body and test equipment and can discharge without detection. Although

the AD7740 features proprietary ESD protection circuitry, permanent damage may occur on

devices subjected to high-energy electrostatic discharges. Therefore, proper ESD precautions are

recommended to avoid performance degradation or loss of functionality.

WARNING!

ESD SENSITIVE DEVICE

–4–

PIN FUNCTION DESCRIPTIONS

8-LEAD microSOIC PIN NUMBERS*

No. Mnemonic Function

1 CLKOUT The crystal/resonator is tied between this pin and CLKIN. In the case of an external clock driving CLKIN, an

inverted clock signal appears on this pin and can be used to drive other circuitry provided it is buffered ﬁrst.

2 CLKIN The master clock for the device may be in the form of a crystal/resonator tied between this pin and CLKOUT.

An external CMOS-compatible clock may also be applied to this input as the clock for the device. If CLKIN

is inactive low for 1 ms (typ), the AD7740 automatically enters power-down.

3 GND Ground reference for all the circuitry on-chip.

4 REFIN/OUT Voltage Reference Input. This is the reference input to the core of the VFC and deﬁnes the span of the VFC.

If this pin is left unconnected, the internal 2.5 V reference is the default reference. Alternatively, a precision

external reference may be used to overdrive the internal reference. The internal reference has high output

impedance in order to allow it to be overdriven.

5 VIN The analog input to the VFC. It has a nominal input range from 0 V to V

REF

which corresponds to an output

frequency of 10% f

CLKIN

to 90% f

CLKIN

. It has a ±150 mV overrange. If buffered, it draws virtually no current

from whatever source is driving it.

6 VDD Power Supply Input. These parts can be operated at 3.3 V ± 10% or 5 V ± 5%. The supply should be

adequately decoupled with a 10 µF and a 0.1 µF capacitor to GND.

7 FOUT Frequency Output. FOUT goes from 10% to 90% of f

CLKIN

, depending on VIN.

8 BUF Buffered Mode Select Pin. When BUF is tied low, the VIN input is unbuffered and the range on the VIN

pin is –0.15 V to VDD + 0.15 V. When it is tied high, VIN is buffered and the range on the VIN pin

is restricted to 0.1 V to VDD – 0.2 V.

*Note that the SOT-23 and microSOIC packages have different pinouts.

ORDERING GUIDE

Package Branding

Model Temperature Range Package Description Option Information

AD7740KRM 0°C to 85°C microSOIC Package RM-8 VOK

AD7740YRT –40°C to +105°C SOT-23 Package RT-8 VOY

AD7740YRM –40°C to +105°C microSOIC Package RM-8 VOY

PIN CONFIGURATIONS

8-Lead microSOIC

microSOIC

TOP VIEW

(Not to Scale)

CLKOUT

CLKIN

GND

REFIN/OUT

BUF

FOUT

VDD

VIN

AD7740

8-Lead SOT-23

SOT-23

TOP VIEW

(Not to Scale)

BUF

FOUT

VDD

VIN

CLKOUT

CLKIN

GND

REFIN/OUT

AD7740

REV. A

AD7740

REV. A –5–

TERMINOLOGY

INTEGRAL NONLINEARITY

For the VFC, Integral Nonlinearity (INL) is a measure of the maxi-

mum deviation from a straight line passing through the actual

endpoints of the VFC transfer function. The error is expressed in

% of the actual frequency span:

Frequency Span = FOUT(max) – FOUT(min)

OFFSET ERROR

Ideally, the output frequency for 0 V input voltage is 10% of

CLKIN

in unbuffered mode. The deviation from this value referred

to the input is the offset error at BUF = 0. In buffered mode the

minimum output frequency (corresponding to 0.10 V minimum

input voltage) is 13.2% of f

CLKIN

at V

REF

= 2.5 V. The deviation

from this value referred to the input is the offset error at BUF = 1.

Offset error is expressed in mV.

GAIN ERROR

This is a measure of the span error of the VFC. The gain is the

scale factor that relates the input VIN to the output FOUT.

The gain error is the deviation in slope of the actual VFC transfer

characteristic from the ideal expressed as a percentage of the full-

scale span. See Figure 2.

OFFSET ERROR DRIFT

This is a measure of the change in Offset Error with changes in

temperature. It is expressed in µV/°C.

GAIN ERROR DRIFT

This is a measure of the change in Gain Error with changes in

temperature. It is expressed in (ppm of span)/°C.

POWER SUPPLY REJECTION RATIO (PSRR)

This indicates how the apparent input voltage of the VFC is

affected by changes in the supply voltage. The input voltage is

kept constant at 2 V, V

REF

is 2.5 V and the VDD supply is varied

⫾10% at 3.3 V and ±5% at 5 V. The ratio of the apparent change

in input voltage to the change in VDD is measured in dBs.

0REFIN

WITH OFFSET

ERROR ONLY

IDEAL

GAIN ERROR

OUTPUT

FREQUENCY

FOUT

0.9 fCLKIN

OFFSET

ERROR

WITH OFFSET

ERROR AND

GAIN ERROR

INPUT

VOLTAGE

VIN

0.1

fCLKIN

Figure 2. Offset and Gain

–6–

–0.015

–0.005

0.005

0.01

–0.01

–0.5 0 0.5 1.5122.5

VIN – V

VDD = 5V

REFIN = 2.5V

CLKIN = 1MHz

= 25C

BUFFER OFF

BUFFER ON

INL ERROR – % of Span

0.015

TPC 1. INL vs. VIN (Buffered and

Unbuffered)

VDD – V

OFFSET ERROR – mV

–803 6

–4

REFIN = 2.5V

CLKIN = 1MHz

= 25C

BUF = 0

GAIN ERROR

OFFSET ERROR

0.08

0.04

–0.12

–0.08

–0.04

GAIN ERROR – % Span

TPC 4. Offset and Gain Error vs. VDD

VDD – V

REFOUT – V

2.520

2.500

2.5 3.0 5.5

3.5

2.515

2.510

TA = 25C

2.505

4.0 4.5 5.0

TPC 7. REFOUT vs. VDD

CLKIN FREQUENCY – kHz

OFFSET ERROR – mV

–4

–5

–80 200 1000

400

–6

–7

600 800

= 5V

REFIN = 2.5V

= 25C

BUFFER ON

BUFFER OFF

TPC 2. Offset Error vs. CLKIN

(Buffered and Unbuffered)

VIN – V

–80

–105

1234

PSRR – dB

–60

–50

–70

VDD = 5V

REFIN = 4.75V

CLKIN = 1MHz

TA = 25C

BUFFER ON

BUFFER OFF

TPC 5. PSRR vs. VIN (Buffered and

Unbuffered)

2.00V CH2 2.00V M 2.00



FOUT

CLKIN

VDD = 5V

REFIN = 5V

= 25C

CLKIN  1 MHz

CH1

TPC 8. Typical FOUT Pulse Train

(VIN = V

REF

/4)

CLKIN FREQUENCY – kHz

–0.1

–0.05

0.05

0.1

200 800600400 1000

GAIN ERROR – % Span

BUFFER

OFF

BUFFER ON

VDD = 5V

REFIN = 2.5V

TA = 25C

TPC 3. Gain Error vs. CLKIN

(Buffered and Unbuffered)

CLKIN FREQUENCY – kHz

– mA

00200

0.25

400 600 800 1000

0.5

0.75

1.25

BUFFER OFF

BUFFER ON

VDD = 5V

REFIN = 5V

TA = 25C

CFOUT = 43pF

CCLKOUT = 22pF

TPC 6. I

vs. CLKIN (Buffered and

Unbuffered)

AD7740–Typical Performance Characteristics

REV. A

AD7740

REV. A –7–

GENERAL DESCRIPTION

The AD7740 is a CMOS synchronous Voltage-to-Frequency

Converter (VFC) which uses a charge-balance conversion

technique. The input voltage signal is applied to a proprietary

front-end based around an analog modulator which converts the

input voltage into an output pulse train.

The part also contains an on-chip 2.5 V bandgap reference and

operates from a single 3.3 V or 5 V supply. A block diagram of

the AD7740 is shown in Figure 3.

VIN

INTEGRATOR

FOUT

COMPARATOR

BUF GND

SWITCHED

CAPS

SWITCHED

CAPS

AD7740

Figure 3. Block Diagram

Input Ampliﬁer Buffering and Voltage Range

The analog input VIN can be buffered by setting BUF = 1. This

presents a high impedance, typically 100 MΩ, which allows

signiﬁcant external source impedances to be tolerated. The VIN

voltage range is now 0.1 V to VDD – 0.2 V. By setting BUF = 0

the AD7740 input circuit accepts an analog input below GND

and the analog input VIN has a voltage range from –0.15 V to

VDD + 0.15 V. In this case the input impedance is typically

650 kΩ.

The transfer function for the AD7740 is represented by:

FOUT = 0.1 f

CLKIN

+ 0.8 (VIN/V

REF

) f

CLKIN

It is shown in Figure 4 for unbuffered mode.

REF

OUTPUT

FREQUENCY

FOUT

REF

+ 0.15V

0.10 f

CLKIN

FOUT MIN INPUT

VOLTAGE

VIN

FOUT MAX

0.90 f

CLKIN

–0.15V

AD7740

Figure 4. Transfer Function

Sample Calculation:

REF

= 2.5 V, BUF = 0

FOUT (min) = 0.1 f

CLKIN

+ 0.8(–0.15/2.5) f

CLKIN

= 0.052 f

CLKIN

FOUT (max) = 0.1 f

CLKIN

+ 0.8(2.65/2.5) f

CLKIN

= 0.948 f

CLKIN

VFC Modulator

The analog input signal to the AD7740 is continuously sampled

by a switched capacitor modulator whose sampling rate is set

by a master clock. The input signal may be buffered on-chip

(BUF = 1) before being applied to the sampling capacitor of the

modulator. This isolates the sampling capacitor charging currents

from the analog input pin.

This system is a negative feedback loop that acts to keep the net

charge on the integrator capacitor at zero, by balancing charge

injected by the input voltage with charge injected by V

REF

. The

output of the comparator provides the digital input for the 1-bit

DAC, so that the system functions as a negative feedback loop

that acts to minimize the difference signal. See Figure 5.

INTEGRATOR

COMPARATOR

1-BIT

BITSTREAM

REF

INPUT

CLK

–V

REF

AD7740

Figure 5. Modulator Loop

The digital data that represents the analog input voltage is con-

tained in the duty cycle of the pulse train appearing at the output

of the comparator. The output is a pulse train whose frequency

depends on the analog input signal. A full-scale input gives an

output frequency of 0.9 f

CLKIN

and zero-scale input gives an

output frequency of 0.1 f

CLKIN

. The output allows simple inter-

facing to either standard logic families or opto-couplers. The

pulsewidth of FOUT is ﬁxed and is determined by the high period

of CLKIN. The pulse is synchronized to the rising edge of the

clock signal. The delay time between the edge of CLKIN and the

edge of FOUT is typically 35 ns. Figure 6 shows the waveform

of this frequency output. (See TPC 8.)

CLKIN

FOUT = f

CLKIN

VIN = V

REF

CLKIN

FOUT = f

CLKIN

VIN = V

REF

FOUT = f

CLKIN

 3/10

VIN = V

REF

CLKIN

AVERAGE FOUT IS f

CLKIN

 3/10 BUT THE ACTUAL PULSE STREAM VARIES

BETWEEN f

CLKIN

/3 and f

CLKIN

Figure 6. Frequency Output Waveforms

If there is a step change in input voltage, there is a settling time

that must elapse before valid data is obtained. This is typically

two CLKIN cycles.

–8–

Clock Generation

As distinct from the asynchronous VFCs that rely on the

stability of an external capacitor to set their full-scale frequency,

the AD7740 uses an external clock to deﬁne the full-scale output

frequency. The result is a more stable transfer function, which

allows the designer to determine the system stability and drift

based upon the selected external clock.

The AD7740 requires a master clock input, which may be an

external CMOS-compatible clock signal applied to the CLKIN

pin (CLKOUT not used). For a frequency of 1 MHz, a crystal

or resonator can be connected between CLKIN and CLKOUT

so that the clock circuit functions as a crystal controlled oscilla-

tor. Figure 7 shows a simple model of this.

CLKIN CLKOUT

5M

C1 C2

ON-CHIP

CIRCUITRY

OFF-CHIP

CIRCUITRY

Figure 7. On-Chip Oscillator

Using the part with a crystal or ceramic resonator between the

CLKIN and CLKOUT pins generally causes more current to

be drawn from VDD than when the part is clocked from a driven

clock signal at the CLKIN pin. This is because the on-chip

oscillator is active in the case of the crystal or resonator. The

amount of additional current depends on a number of factors.

First, the larger the value of the capacitor on CLKIN and

CLKOUT pins, the larger the current consumption. Typical

values recommended by the crystal and resonator manufacturers

are in the range of 30 pF to 50 pF. Another factor that influ-

ences I

is Effective Series Resistance of the crystal (ESR).

The lower the ESR value, the lower the current taken by the

oscillator circuit.

The on-chip oscillator also has a start-up time associated with it

before it oscillates at its correct frequency and voltage levels. The

typical start-up time is 10 ms with a V

of 5 V and 15 ms with

a V

of 3.3 V (both with a 1 MHz crystal).

The AD7740 master clock appears inverted on the CLKOUT

pin of the device. The maximum recommended load on this pin is

one CMOS load. When using a crystal to generate the AD7740’s

clock it may be desirable to then use this clock as the clock

source for the entire system. In this case, it is recommended that

the CLKOUT signal be buffered with a CMOS buffer before

being applied to the rest of the circuit (as shown in Figure 7).

Reference Input

The AD7740 performs conversions relative to the applied refer-

ence voltage. This reference may be taken from the internal 2.5 V

bandgap reference by leaving REFIN/OUT unconnected. Alterna-

tively an external precision reference may be used. This is

connected to the REFIN/OUT pin, overdriving the internal

reference. Drive capability, initial error, noise, and drift charac-

teristics should be considered when selecting an external refer-

ence. The AD780 and REF192 are suitable choices for external

references.

The internal reference is most suited to applications where

ratiometric operation of the signal source is possible. Using the

internal reference in systems where the signal source varies with

time, temperature, loading, etc., tends to cancel out errors.

Power-Down Mode

When CLKIN is inactive low for 1 ms (typ), the AD7740 auto-

matically enters a power-down mode. In this mode most of the

digital and analog circuitry is shut down and REFOUT floats.

FOUT goes high. This reduces the power consumption to 525 µW

max (5 V) and 360 µW (3.3 V).

APPLICATIONS

The basic connection diagram for the part is shown in Figure 8.

In the connection diagram shown, the AD7740 is conﬁgured in

unbuffered mode. The 5 V power supply is used as a reference to

the AD7740. A quartz crystal provides the master clock source

for the part. It may be necessary to connect capacitors (C1 and

C2 in the diagram) to the crystal to ensure that it does not oscil-

late at overtones of its fundamental operating frequency. The

values of capacitors will vary depending on the manufacturer’s

speciﬁcations.

REFIN

AD7740

CLKIN CLKOUT

C1 C2

FOUT

GND

BUF

0.1F10F

VIN

VDD

Figure 8. Basic Connection Diagram

AD7740

REV. A

AD7740

REV. A –9–

A/D Conversion Techniques Using the AD7740

One method of using a VFC in an A/D system is to count the

output pulses of FOUT for a ﬁxed gate interval (see Figure 9).

This ﬁxed gate interval should be generated by dividing down

the clock input frequency. This ensures that any errors due to

clock jitter or clock frequency drift are eliminated. The ratio of

the FOUT frequency to the clock frequency is what is important

here, not the absolute value of FOUT. The frequency divi-

sion can be done by a binary counter where CLKIN is the

counter input.

FREQUENCY

DIVIDER

GATE

SIGNAL

FOUT

VIN TO P

AD7740 COUNTER

CLOCK

GENERATOR

CLKIN

Figure 9. A/D Conversion Using the AD7740 VFC

Figure 10 shows the waveforms of CLKIN, FOUT, and the

Gate signal. A counter counts the rising edges of FOUT while the

Gate signal is high. Since the gate interval is not synchronized with

FOUT, there is a possibility of a counting inaccuracy. Depending

on FOUT, an error of one count may occur.

GATE tGATE

FOUT

CLKIN

Figure 10. Waveforms in an A/D Converter Using a VFC

The clock frequency and the gate time determine the resolution

of such an ADC. If 12-bit resolution is required and CLKIN is

1 MHz (therefore, FOUT

MAX

is 0.9 MHz), the minimum gate

time required is calculated as follows:

N counts at Full Scale (0.9 MHz) will take

(N/0.9 × 10

) seconds = minimum gate time

N is the total number of codes for a given resolution; 4096 for

12 bits.

minimum gate time = (4096/0.9 × 10

) seconds = 4.551 ms

Since T

GATE

× FOUT

MAX

= number of counts at full scale, the

fastest conversion for a given resolution can be performed with

the highest CLKIN frequency.

If the output frequency is measured by counting pulses gated to

a signal derived from the clock, the clock stability is unimportant

and the device simply performs as a voltage-controlled frequency

divider, producing a high-resolution ADC. The inherent mono-

tonicity of the transfer function and wide range of input clock

frequencies allows the conversion time and resolution to be

optimized for speciﬁc applications.

Another parameter is taken into account when choosing the

length of the gate interval. Because the integration period of the

VFC is equal to the gate interval, any interfering signal can be

rejected by counting for an integer number of periods of the

interfering signal. For example, a gate interval of 100 ms will

give normal-mode rejection of 50 Hz and 60 Hz signals.

Isolation Applications

The AD7740 can also be used in isolated analog signal trans-

mission applications. Due to noise, safety requirements or distance,

it may be necessary to isolate the AD7740 from any controlling

circuitry. This can easily be achieved by using opto-isolators.

This is extremely useful in overcoming ground loops between

equipment.

The analog voltage to be transmitted is converted to a pulse

train using the VFC. An opto-isolator circuit is used to couple

this pulse train across an isolation barrier using light as the

connecting medium. The input LED of the isolator is driven

from the output of the AD7740. At the receiver side, the output

transistor is operated in the photo-transistor mode. The pulse

train can be reconverted to an analog voltage using a frequency-

to-voltage converter; alternatively, the pulse train can be fed into

a counter to generate a digital signal.

The analog and digital sections of the AD7740 have been designed

to allow operation from a single-ended power source, simplify-

ing its use with isolated power supplies.

Figure 11 shows a general purpose VFC circuit using a low cost

opto-isolator. A 5 V power supply is assumed for both the iso-

lated (V

) and local (V

) supplies.

VIN FOUT

GND1

VCC

GND2

ISOLATION

BARRIER

0.1F10F

AD7740

VDD

OPTOCOUPLER

Figure 11. Opto-Isolated Application

–10–

Temperature Sensor Application

The AD7740 can be used with an AD22100S temperature

sensor to give a digital measure of ambient temperature. The

output voltage of the AD22100S is proportional to the tempera-

ture times the supply voltage. It uses a single 5 V supply, and its

output swings from 0.25 V at –50°C to 4.75 V at +150°C. By

feeding its output through the AD7740, the value of ambient

temperature is converted into a digital pulse train. See Figure 12.

REFIN

AD7740

AD22100S

V+

CLKIN CLKOUT

C1 C2

FOUT

GND

BUF

0.1F10F 0.1F

VIN

VDD

10F

Figure 12. Using the AD7740 with a Temperature Sensor

Due to its ratiometric nature this application provides an

extremely cost-effective solution. The need for an external pre-

cision reference is eliminated since the 5 V power-supply is used

as a reference to both the VFC and the AD22100S.

32 kHz Operation

The AD7740 oscillator circuit will not operate at 32 kHz. If

the user wishes to use a 32 kHz watch crystal, some additional

external circuitry is required. The circuit in Figure 13 is for a

crystal with a required drive of 1 µW. Resistors R1 and R2

reduce the power to this level.

1M

CLKIN

100k

220k

32kHz

40106 40106

Figure 13. 32 kHz Watch Crystal Circuit

Power Supply Bypassing and Grounding

In any circuit where accuracy is important, careful consideration

of the power supply and ground return layout helps to ensure

the rated performance. The printed circuit board housing the

AD7740 should be designed such that the analog and digital

sections are separated and conﬁned to certain areas of the board.

To minimize capacitive coupling between them, digital and

analog ground planes should only be joined in one place, close

to the AD7740, and should not overlap.

Avoid running digital lines under the device, as these will couple

noise onto the die. The analog ground plane should be allowed

to run under the AD7740 to avoid noise coupling. The power

supply lines to the AD7740 should use as large a trace as pos-

sible to provide low impedance paths and reduce the effects of

glitches on the power supply line. Fast switching signals like

clocks should be shielded with digital ground to avoid radiating

noise to other parts of the board, and clock signals should never

be run near analog inputs. Avoid crossover of digital and analog

signals. Traces on opposite sides of the board should run at right

angles to each other. This reduces the effect of feedthrough

through the board. A microstrip technique is by far the best but

is not always possible with a double-sided board. In this technique,

the component side of the board is dedicated to the ground plane

while the signal traces are placed on the solder side.

Good decoupling is also important. All analog supplies should

be decoupled to GND with surface mount capacitors, 10 µF in

parallel with 0.1 µF located as close to the package as possible,

ideally right up against the device. The lead lengths on the by-

pass capacitor should be as short as possible. It is essential that

these capacitors be placed physically close to the AD7740 to

minimize the inductance of the PCB trace between the capacitor

and the supply pin. The 10 µF are the tantalum bead type and

are located in the vicinity of the VFC to reduce low-frequency

ripple. The 0.1 µF capacitors should have low Effective Series

Resistance (ESR) and Effective Series Inductance (ESI), such

as the common ceramic types, which provide a low imped-

ance path to ground at high frequencies to handle transient

currents due to internal logic switching. Additionally, it is ben-

eﬁcial to have large capacitors (> 47 µF) located at the point

where the power connects to the PCB.

AD7740

REV. A

AD7740

REV. A –11–

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

8-Lead microSOIC

(RM-8)

0.011 (0.28)

0.003 (0.08)

0.028 (0.71)

0.016 (0.41)

33

27

0.120 (3.05)

0.112 (2.84)

0.122 (3.10)

0.114 (2.90)

0.199 (5.05)

0.187 (4.75)

PIN 1

0.0256 (0.65) BSC

0.122 (3.10)

0.114 (2.90)

SEATING

PLANE

0.006 (0.15)

0.002 (0.05)

0.018 (0.46)

0.008 (0.20)

0.043 (1.09)

0.037 (0.94)

0.120 (3.05)

0.112 (2.84)

8-Lead SOT-23

(RT-8)

1234

8765

0.122 (3.10)

0.110 (2.80)

PIN 1

0.077 (1.95)

BSC

0.026 (0.65) BSC

0.071 (1.80)

0.059 (1.50)

0.009 (0.23)

0.003 (0.08)

0.022 (0.55)

0.014 (0.35)

10

0

0.015 (0.38)

0.009 (0.22)

0.006 (0.15)

0.000 (0.00)

0.051 (1.30)

0.035 (0.90)

SEATING

PLANE

0.057 (1.45)

0.035 (0.90)

C01030a–2.5–2/01 (rev. A)

PRINTED IN U.S.A.