TABLE 2. Input to Output Relationship – Single-Ended
Input
VIN+VIN−Output
VCM − VREF VCM 0000 0000 0000
VCM − VREF/2 VCM 0100 0000 0000
VCM VCM 1000 0000 0000
VCM + VREF/2 VCM 1100 0000 0000
VCM +VREF VCM 1111 1111 1111
1.3.1 Single-Ended Operation
Single-ended performance is lower than with differential input
signals. For this reason, single-ended operation is not rec-
ommended. However, if single-ended operation is required,
and the resulting performance degradation is acceptable, one
of the analog inputs should be connected to the d.c. common
mode voltage of the driven input. The peak-to-peak differen-
tial input signal should be twice the reference voltage to
maximize SNR and SINAD performance (Figure 2b). For ex-
ample, set VREF to 1.0V and bias VIN− to 1.0V and drive VIN+
with a signal range of 0V to 2.0V.
Because very large input signal swings can degrade distortion
performance, better performance with a single-ended input
can be obtained by reducing the reference voltage while
maintaining a full-range output. and indicate the input to out-
put relationship of the ADC12040.
1.3.2 Driving the Analog Inputs
The VIN+ and the VIN− inputs of the ADC12040 consist of an
analog switch followed by a switched-capacitor amplifier. The
capacitance seen at the analog input pins changes with the
clock level, appearing as 8 pF when the clock is low, and 7
pF when the clock is high. Although this difference is small, a
dynamic capacitance is more difficult to drive than is a fixed
capacitance, so choose the driving amplifier carefully. The
LMH6550, the LMH6702 and the LMH6628 are a good am-
plifiers for driving the ADC12040.
The internal switching action at the analog inputs causes en-
ergy to be output from the input pins. As the driving source
tries to compensate for this, it adds noise to the signal. To
prevent this, use an RC at each of the inputs, as shown in
Figure 5 and Figure 6. These components should be placed
close to the ADC because the input pins of the ADC is the
most sensitive part of the system and this is the last opportu-
nity to filter the input. The capacitors are for Nyquist applica-
tions and should be eliminated for undersampling applica-
tions.
The LMH6550 and the LMH6552 are excellent devices for
driving the ADC12040, especially when single-ended to dif-
ferential conversion with d.c. coupling is necessary. An ex-
ample of the use of the LMH6550 to drive the analog input of
the ADC12040 is shown in Figure 5.
For high frequency, narrow band applications, a transformer
is generally the recommended way to drive the analog inputs,
as shown in Figure 6.
1.3.3 Input Common Mode Voltage
The input common mode voltage, VCM, should be in the range
indicated in Section 1.0 and be of a value such that the peak
excursions of the analog input signal do not go more negative
than ground or more positive than the VA supply voltage. The
nominal VCM should generally be equal to VREF/2, but VRM can
be used as a VCM source as long as VRM need not supply more
than 10 µA of current. Figure 5 shows the use of the VRM out-
put to drive the VCM input of the LMH6550. The common mode
output voltage of the LMH6550 is equal to the VCM input input
voltage.
2.0 DIGITAL INPUTS
The digital TTL/CMOS compatible inputs consist of CLK,
OE and PD.
2.1 The CLK Input
The CLK signal controls the timing of the sampling process.
Drive the clock input with a stable, low jitter clock signal in the
range of 100 kHz to 50 MHz with rise and fall times of less
than 3ns. The trace carrying the clock signal should be as
short as possible and should not cross any other signal line,
analog or digital, not even at 90°.
If the CLK is interrupted, or its frequency too low, the charge
on internal capacitors can dissipate to the point where the
accuracy of the output data will degrade. This is what limits
the lowest sample rate to 100 ksps.
The duty cycle of the clock signal can affect the performance
of the A/D Converter. Because achieving a precise duty cycle
is difficult, the ADC12040 is designed to maintain perfor-
mance over a range of duty cycles. While it is specified and
performance is guaranteed with a 50% clock duty cycle, per-
formance is typically maintained over a clock duty cycle range
of 45% to 55%.
The clock line should be terminated at its source in the char-
acteristic impedance of that line. It is highly desirable that the
the source driving the ADC CLK input only drive that pin.
However, if that source is used to drive other things, each
driven pin should be a.c. terminated with a series RC to
ground, as shown in Figure 4, such that the resistor value is
equal to the characteristic impedance of the clock line and the
capacitor value is
where tPD is the signal propagation rate down the clock line,
"L" is the line length and ZO is the characteristic impedance
of the clock line. This termination should be as close as pos-
sible to the ADC clock pin but beyond it as seen from the clock
source. Typical tPD is about 150 ps/inch (60 ps/cm) on FR-4
board material. The units of "L" and tPD should be the same
(inches or centimeters).
Take care to maintain a constant clock line impedance
throughout the length of the line. Refer to Application Note
AN-905 or AN-1113 for information on setting and determin-
ing characteristic impedance
2.2 The OE Input
The OE input, when high, puts the output pins into a high
impedance state. When this pin is low the outputs are in the
active state. The ADC12040 will continue to convert whether
this input is high or low, but the output can not be read while
the OE pin is high.
The OE input should NOT be used to multiplex devices to-
gether to drive a common bus as this will result in excessive
capacitance on the data output pins, reducing SNR and
SINAD performance of the converter. See Section 3.0.
2.3 The PD Input
The PD input, when high, holds the ADC12040 in a power-
down mode to conserve power when the converter is not
being used. The power consumption in this state is 70 mW
with a 40MHz clock and 40mW if the clock is stopped. The
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ADC12040