Application Note 80 - Linear Technology

Application Note 80

AN80-1

July 1999

How to Use the World’s Smallest 24-Bit

No Latency Delta-SigmaTM ADC to its Fullest Potential

Frequently Asked Questions About Delta-Sigma ADCs and the LTC2400

By Michael K. Mayes

Linear Technology’s LTC

2400 is the world’s first 24-bit

ADC in an SO-8 Package. An innovative new delta-sigma

architecture has been developed. The result is a small,

highly accurate, simple-to-use delta-sigma ADC. This

paper was created to educate users on several topics

associated with delta-sigma converters and to dispel

confusion associated with this new one-shot, or No

Latency ∆Σ

, architecture. The key topics addressed here

include speed, noise, PGAs, line frequency, rejection,

input current, multiplexing, analog input range and key

features differentiating the LTC2400 from other delta-

sigma ADCs.

, LTC and LT are registered trademarks of Linear Technology Corporation.

No Latency Delta-Sigma and No Latency ∆Σ are trademarks of Linear Technology Corporation.

Application Note 80

AN80-2

LTC2400 FREQUENTLY ASKED QUESTIONS

I. Questions Dealing with Speed

1. I currently use a competitor’s delta sigma running at an

output rate of 60Hz; can I run the LTC2400 at 60Hz?

Competition: For a 60Hz notch, most of the competition’s

output rates are 60Hz. However, 3 out of 4 output data are

redundant; therefore these results can be thrown away.

This, combined with the overhead of calibration and filter

settling (up to an additional 3 conversion cycles), reduces

the competitors’ effective output rate below that of the

LTC2400.

LTC2400: The output rate is defined by the notch fre-

quency (f

) divided by 8. For example, with a notch

frequency of 60Hz, the output rate of the LTC2400 is

7.5Hz. Each data output contains nonredundant data.

Additionally, the full scale and offset are calibrated trans-

parently to the user for each conversion cycle. The LTC2400

combines all the data into one highly accurate result with

much more rejection than the competition. There is no

need for the user to sift through redundant data because

the filter settles in a single conversion cycle.

2. I currently use a competitor’s part running at a 1kHz

data output rate with averaging to achieve low noise.

Can the LTC2400 run at 1kHz output rate?

No, but for this application, the LTC2400 running at 7.5Hz

output rate offers lower noise performance than the

competition’s running at 1kHz with averaging. Addition-

ally, the competitors part running at 1kHz does not provide

rejection of line frequencies (50Hz/60Hz); the user is

required to add an external digital lowpass FIR filter to

reject line frequency noise.

Competition: Running at 1kHz output rate, the peak-to-

peak resolution degrades to 10 bits–12 bits (from

16 bits–20 bits). Averaging the 1kHz output rate 128 times

improves the noise performance 3.5 bits. The final effec-

tive resolution is only increased to 13.5 bits to 15.5

bits.

Additionally, the competitors’ part does not reject

50Hz/60Hz.

LTC2400: Running at a 7.5Hz output rate, the LTC2400’s

noise performance is 1.5µV

RMS

(21.6 bits). Additionally,

the LTC2400 rejects line frequency noise (50Hz/60Hz +

harmonics) by 120dB, without the need for an external

DSP. The LTC2400 offers a highly accurate one-shot

result, removing the burden of external averaging or

digital filtering.

3. What applications are suitable for the LTC2400?

The LTC2400 is the ideal converter for any application

requiring high DC accuracy. These applications include

DC voltage/current measurements, gas analysis, weigh

scales, temperature measurements (thermocouples, RTDs,

thermistors), battery charging/monitoring, portable hand-

held instrumentation, smart transmitters, DC multiplexed

data acquisition and digital panel meters.

4. What applications are not intended for the LTC2400?

Digital audio, seismic and signal acquisition applications.

5. What is the maximum conversion rate I can achieve

with the LTC2400?

The maximum conversion rate for this particular part is

15Hz (with 120Hz rejection).

6. What does the FFT of the conversion result look like?

There is no FFT associated with the LTC2400. Each con-

version is a single-shot result, statistically independent

from previous conversion cycles. It is a true DC accurate

converter. The filter response may be determined by

sweeping the input frequency and calculating the RMS

noise associated with the corresponding output.

Application Note 80

AN80-3

II. Questions Dealing with Noise and PGAs

1. Do I need a PGA with the LTC2400 (the competitive

parts have built-in PGAs)?

No, a PGA is not required with the LTC2400.

Competition: Competitor’s ADCs require complex pro-

gramming of status registers, flushing of filters between

PGA gain changes, and are limited to a reduced input range

of 0V to V

REF

divided by the PGA gain.

LTC2400: The ultralow noise performance of the LTC2400

(0.3ppm RMS) combined with total unadjusted errors

less than 10ppm enable direct digitizing of low level

signals. The input range is not limited to VREF/PGA gain;

the part gives equivalent performance over a much wider

input range of (– 0.125 • VREF) to (1.125 • VREF).

2. How does the noise performance of the LTC2400 com-

pare to that of other delta-sigma converters?

LTC2400: The LTC2400 is the world’s quietest delta-

sigma ADC. With an input range of 0V to 5V, the RMS noise

is 0.3ppm or 1.5µV

RMS

. This translates to an effective

resolution of 21.6 bits.

Competition: The best competition noise performance is

5.3µV

RMS

(or 19.8 bits) on a ±2.5V input.

3. How does the LTC2400 noise performance change with

input voltage?

Competition: Many low noise delta-sigma converters use

second order modulators. This approach results in a

phenomenon known as fixed pattern noise. As a result, the

RMS noise performance of the device depends on the

input voltage. The user sees sparkle codes or large noise

spikes in the conversion results. The part behaves worse

than the specifications state.

LTC2400: The LTC2400 uses a 3rd-order modulator

instead of a 2nd-order modulator. This results in noise

performance of 1.5µV

RMS

, independent of the input

voltage.

4. What is the effect on noise if I lower V

REF

The noise in µV

RMS

is independent of V

REF

. However, the

noise in LSB or ppm of full scale is inversely proportional

with the reference. For every halving of V

REF

, the noise in

ppm of full scale doubles.The noise at V

REF

= 5V is 0.3ppm

RMS and at V

REF

= 2.5V is 0.6ppm RMS.

5. How do I measure a small voltage range (100mV)

sitting on top of a large DC offset (several volts)?

Competition: The competition limits the maximum input

voltage to V

REF

divided by the PGA gain. As a result,

digitizing microvolts sitting on top of volts is difficult. One

method requires high PGA gain and an external analog

circuit to level shift the input down to 0V from the large DC

offset. A second method requires a PGA gain of 1 at the

expense of the large RMS noise and large INL errors.

LTC2400: Simply digitize directly. The noise of the LTC2400

is 1.5µV

RMS

. Additionally, the total unadjusted error of the

LTC2400 is 10ppm (many times better then the competi-

tion). This enables very accurate measurements of a

microvolt input signals independent of the large DC offset

voltage.

Application Note 80

AN80-4

III. Questions Dealing with Line-Frequency Rejection

1. What is the rejection with 2% variations in line

frequency?

Competition: The competition uses 3rd-order sinc (sinx/x)

filters. The rejection at ±2% is 100dB even with a very

precise external clock (i.e., very low jitter).

LTC2400: The LTC2400 combines a 4th-order sinc filter

with a highly accurate, on-chip, low drift oscillator. The

filter rejection is 120dB for 60Hz ±2% without requiring

any external frequency-setting components.

2. How accurate a clock do I need to generate for my own

notch frequency other than 50Hz or 60Hz?

Competition: Other delta-sigma converters incorporate a

1st- or 3rd-order sinc filter. A 3rd-order sinc filter requires

the external clock accuracy within ±1% to reject 120dB

and ±2% to reject 100dB.

LTC2400: As a result of the 4th-order sinc filter, an

external clock can vary up to ±3% and the filter will still

reject 120dB. Variations of 5% will still reject >100dB.

3. What value external clock frequency do I need to apply

for an 8Hz rejection?

The relationship between notch frequency (f

) and the

externally applied clock frequency (f

EXT

) is f

EXT

= 2560 • f

Therefore, an 8Hz notch frequency is achieved by applying

a 20,480Hz clock at pin f

4. How can I get 120dB rejection for 50Hz and 60Hz

simultaneously?

Set the notch frequency to 10Hz (f

EXT

= 25.600kHz). Since

the sinc filter rejects the notch frequency (f

) and its

harmonics, both 50Hz and 60Hz are rejected 120dB.

5. For a 60Hz rejection frequency, what is the rejection at

120Hz?

The rejection at 120Hz is in excess of 120dB. The sinc filter

rejects 60Hz (f

) and all its harmonics up to the internal

sampling frequency (f

= 256 • f

= 15,360Hz).

6. What are the effects of aliasing?

One of the advantages of delta-sigma converters is the

reduction in antialiasing filter complexity. Typically, a

simple single-pole filter at the input is sufficient to remove

aliasing components at the multiples of the internal sample

rate (f

= 256 • f

= 15,360Hz). This is a common feature

of the LTC2400 and other delta-sigma ADCs.

7. If I externally set the notch frequency to 120Hz, what is

the rejection at 60Hz?

Delta-sigma ADCs do not significantly reject frequencies

below the first notch frequency.

Competition: The rejection is 11.7dB at 60Hz (sinc

filter).

LTC2400: The rejection is 15.9dB at 60Hz (sinc

filter).

8. What is the rejection of a 60Hz signal applied to the

power supply pin? The V

REF

Pin?

Competition: In order for the competition to achieve their

noise performance, several supply and ground pins are

required. This adds complexity to the user’s board in

terms of supply/ground routing and bypassing.

LTC2400: The LTC2400 has one ground pin common to

the input, reference, supply, and digital I/O reducing layout

board area. The single supply and the reference pin reject

60Hz noise better than 120dB.

Application Note 80

AN80-5

IV. Questions Dealing with Multiplexing

1. Is it possible to put a multiplexer in front of a delta-

sigma ADC?

Competition: Due to the long digital filter settling times

associated with these converters, multiplexing is difficult

and time consuming.

LTC2400: Simple. Convert, read the result, change the

channel, convert, read the result, change the channel,...

There is no difference between the LTC2400 and conven-

tional SAR ADCs with respect to multiplexing. The LTC2400

utilizes a single-shot conversion, or No Latency ∆Σ,

architecture.

2. What is the settling time of the converter?

Competition: Other delta-sigma converters require the

internal digital filter to accumulate data for several conver-

sion cycles. This forces the user to discard the first 3 or 4

conversion results after the input is changed or a multi-

plexer is switched.

LTC2400: The LTC2400 has no filter settling time. There is

a one-to-one correspondence between the conversion

result and the analog input. As a result the LTC2400 is

simple to multiplex.

V. Questions Dealing with Input Signal Range

1. What is the input range of the LTC2400?

Competition: The competition limits the input range from

0V to V

REF

, for single supply operation. For a 2.5V refer-

ence, the input range is 0V to 2.5V.

LTC2400: On a single supply, the LTC2400 can convert

input signals 12.5% of V

REF

below ground up to 12.5%

above V

REF

. With a 2.5V reference, the input range is

–300mV to 2.8V. With a 5V reference, the input range is

–300mV to 5.3V.

2. How far below ground does the LTC2400 still accurately

convert?

Competition: Competitors delta-sigma converters, utiliz-

ing a single supply, cannot convert below ground (0V) or

require external charge pump circuitry.

LTC2400: The LTC2400 is live at zero. The input can go as

low as 300mV below ground (even with a single 5V

supply) and 300mV above V

REF

3. In the extended input range, what is the data out

format?

Competition: Does not allow the user to drive the inputs

above V

REF

or below GND. They either become unstable or

clamp the outputs.

LTC2400: Within the normal input voltage range (V

= 0V

to V

REF

) the digital output ranges from 00000

to FFFFF

If the analog input voltage exceeds V

REF

, an overrange bit

is set and the remaining 24 bits correspond to the mea-

sured overvoltage. If the analog input is below ground, a

sign bit is set high and the digital output becomes two’s

compliment indicating the measured voltage below GND.

4. What happens to the digital output code if the extended

input range is exceeded?

The digital output remains constant at 12.5% above full

scale or below ground. Beyond 300mV above V

or below

ground, the input ESD protection diodes begin to forward

bias. In extreme cases where the voltage applied to the

LTC2400 can exceed these limits, the input current can be

limited with an external resistor up to 10k.

Application Note 80

AN80-6

VI. Questions Dealing with Input Impedance.

1. What does the input impedance look like?

Competition: Some competitive parts have very high input

impedance, and near zero input current.These parts use an

internal buffer to isolate the switched-capacitor modulator

from the input. These buffers result in major limitations on

input swing (50mV above ground and 1.5V below V

Most applications do not use this buffer due to its limited

input range and output stage’s inability to pull to GND. The

resulting input impedance without this buffer is similar to

that of the LTC2400.

LTC2400: The input to the LTC2400 is a fixed 10pF

capacitor, and a dynamic switched-capacitor load equiva-

lent to 1.66MΩ.

2. What is the effect of the input impedance if I have an

external resistance in series with the device?

Competition and LTC2400: An external source resistance

increases the RC time constant associated with charging

the ADC’s internal capacitor. In order to achieve 20 bits of

accuracy, the input signal applied to the ADC must settle

to 14 time constants within the sampling window.

The LTC2400 has an internal sampling capacitor of 10pF

and a parasitic input resistance of 5k. The sampling

window is 6.5µs. Therefore, the maximum time constant

allowed is 6.5µs/14 = 464ns. The resulting maximum

external source resistance is 464ns/10pF – 5k = 41.4k.

This is an approximation neglecting the effects of input

parasitic capacitance. Practically, source resistors up to

10k can be used.

3. If the external source resistance is nonlinear, what is the

effect on the linearity of the ADC?

As long as the input settles to 14 time constants within

6.5µs, the input source resistance nonlinearities have no

effect on the ADC linearity.

4. What is the effect of the input impedance if an RC filter

is used on the input with an external RC time constant

greater than 6.5

s/14 time constants?

This results in a overall system gain error. This error is zero

at V

= V

REF

/2, maximum at V

= V

REF

. For every 3Ω of

input source resistance, this error is 1ppm at V

= V

REF

and –1ppm at V

= 0V.

Application Note 80

AN80-7

VII. Miscellaneous

1. How does the performance change with supply

voltage?

Competition: These delta-sigma converters cannot guar-

antee performance over the entire 2.7V to 5.5V supply

range. They typically specify two separate parts, one

guaranteed 2.7V to 3.3V and the other 4.5V to 5.5V.

LTC2400: Its performance characteristics are guaranteed

for supply voltages from 2.7V to 5.5V, inclusive.

2. What is the effect on INL if I lower V

REF

The INL performance is improved as V

REF

is reduced. At

REF

= 5V, the INL error is 4ppm (0.0004%). At V

REF

2.5V, the INL is 2ppm (0.0002%).

3. How was Linear Technology able to squeeze the LTC2400

into an SO-8 package when no one else could?

Competition: 24-pin DIPs, 28-lead SO, multiple power

supply pins, complex user interfaces and external crystals.

LTC2400: The advantage the LTC2400 has over the com-

petitors is an innovative, simple digital sinc filter architec-

ture. This enables the combination of a small die size with

an analog optimized process. The result is a highly accu-

rate, S0-8 package, delta-sigma ADC not requiring an

external crystal.

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

4. What are the resolution, linearity and accuracy of the

LTC2400 compared to those of other delta-sigma

converters?

Resolution: Resolution is typically defined as the noise

performance of the ADC in bits. The resolution in bits is:

Resolution = Log

REF

/RMSnoise)/Log

(2). The

LTC2400 is 21.6 bits and the competition is all below 20

bits. Some competitors claim more than 20 bits; how-

ever, they are using a 2nd-order modulator, so the real

noise seen by the user is significantly higher than the

specification seen in the data sheet.

Linearity: Linearity is defined as the deviation from a

straight line drawn from V

REF

= 0V to V

= V

REF

(ignoring

offset and full-scale errors). The LTC2400 has a linearity

error of 4ppm (18 bits) compared to 30ppm (15 bits) for

the competition. Note: 1ppm = 1part per million = 0.0001%

Accuracy: Accuracy is defined as the offset error + full-

scale error + linearity error + noise + drift. The LTC2400 is

the world’s most accurate No Latency ∆Σ

ADC. The offset

error is less than 1ppm. The drift of the offset with

temperature is less than 0.01ppm/°C. The full-scale error

is 4ppm, with a drift of 0.02ppm/°C. The linearity is 4ppm,

and the noise is 0.3ppm RMS.

Application Note 80

AN80-8

VII. Key Features Differentiating the LTC2400 from

the Competition

1. S0-8 Package—The LTC2400 is the smallest 24-bit

ADC on the market.

2. Absolute Accuracy —The total unadjusted error (TUE)

of the LTC2400 is less than 10ppm over 2.7V to 5.5V

supply and –45°C to 85°C operation.

3. On-Chip Oscillator—The LTC2400 does not require

external crystals or oscillators.

4. Live at Zero—The LTC2400 continues to resolve

signals below ground up to –12.5% of V

REF

5. Overrange—The LTC2400 continues to resolve sig-

nals above V

REF

up to 12.5% of V

REF

6. High Accuracy INL—The integral nonlinearity of the

LTC2400 is 4ppm with a 5V reference and 2ppm with

a 2.5V reference

7. High Accuracy DNL—The LTC2400 outputs 24 bits

with no missing codes (guaranteed monotonic).

8. Low Noise—0.3ppm RMS (1.5µV

RMS

) noise.

9. Ultralow Offset—The offset of the LTC2400 is within

1ppm.

10. Very Low Offset Drift—The offset of the LTC2400

drifts less than 1ppm over the entire temperature

range – 45°C to 85°C. This corresponds to

0.01ppm/°C drift. No external calibration is required.

11. Very Low Full-Scale Error—The full-scale error of the

LTC2400 is within 4ppm. No external calibration is

required.

12. Low Full-Scale Drift—The full scale of the LTC2400

drifts less than 2ppm over the entire temperature

range –45°C to 85°C. This corresponds to

0.02ppm/°C drift.

13. No Calibration Required—The LTC2400 performs off-

set/full-scale calibration transparently to the user.

Calibration is interleaved continuously within the con-

version cycle.

14. Pin Selectable 50Hz/60Hz Notch Frequency—The

LTC2400 internal oscillator can be set to reject 50Hz

or 60Hz by simply tying the f

pin High (50Hz) or Low

(60Hz).

15. Simple to Use—The LTC2400 does not contain the

overhead of status registers and configuration

registers.

16. Superior Rejection—The LTC2400 contains a 4th-

order sinc filter. This allows 120dB rejection of line

frequencies ±2%, with an on-chip highly accurate

oscillator.

17. Flexible Reference Input—V

REF

may be equal to V

as low as 0.1V.

18. Low Supply Current—The LTC2400 consumes 200µA

during conversion and 20µA during autoshutdown.

19. Wide Supply Range—The LTC2400 operates from

2.7V to 5.5V supply range.

an80 LT/TP 0799 4K • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 1999

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900

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FAX: (408) 434-0507

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