Texas Instruments | Setting a Constant Common-Mode Voltage for Fully-Differential SAR ADC Inputs | Application notes | Texas Instruments Setting a Constant Common-Mode Voltage for Fully-Differential SAR ADC Inputs Application notes

Texas Instruments Setting a Constant Common-Mode Voltage for Fully-Differential SAR ADC Inputs Application notes
Adjusting the Input Common-Mode Voltage for SAR ADCs
to Avoid Amplifier Output Swing Limitations
Ryan Andrews
TI suggests pairing a fully-differential SAR ADC with a
fully-differential amplifier (FDA) to achieve a constant
common-mode voltage. FDAs feature an output
common-mode pin (VOCM) that sets the common-mode
voltage for both the inverting and non-inverting
outputs. Applying VREF/2 to the VOCM pin allows the
FDA to accept either a single-ended or differential
input and convert it to a symmetrical, differential output
centered around VREF/2.
V+
VS+
AVDD
+
FDA
VS-
VOCM
±
AIN_P
SAR ADC
AIN_N
VREF
AVSS
Figure 1. Setting FDA VOCM = ADC VREF/2
Amplifier Output Swing Limitations
Most SAR and FDA signal chains share the same
positive and negative supply voltages. However, the
ADC reference voltage is usually less than the supply,
so a mid-reference input common-mode for the ADC is
often less than mid-supply of the amplifier. This forces
the output of the FDA to swing closer to the negative
supply rail than to the positive supply rail.
Amplifiers with limited output swing will enter a nonlinear region of operation and introduce additional
error for large output signals. Higher output load
currents further restrict the amplifier output swing, as
load currents can spike for brief instances when the
voltage is sampled by a switched-capacitor SAR ADC.
Ultimately, these limitations may lead to poor linear
performance when the amplifier output signal reaches
the ADC full-scale.
Each of the above solutions require additional
components and add to the design cost, complexity,
and size. A negative supply rail may require an
inverting DC/DC converter plus additional passive
components. The shifted common-mode voltage must
be buffered because the VOCM pin on an FDA is a
relatively low-impedance input.
ADS9224R Fully-Differential SAR ADC
The ADS9224R is a 16-bit, 3-MSPS, dual-channel,
simultaneous-sampling SAR ADC with two key
features that address this very challenge. This device
integrates a buffered mid-reference output that can
drive the VOCM pin directly to VREF/2. Setting a single bit
in the ADS9224R register map increases the VREF/2
output by +100 mV. For example, if the reference
voltage is 4.096 V, the VREF/2 output can be set to
either 2.048 V (default) or to 2.148 V. At the same
time, the input common-mode voltage range on the
ADS9224R has been extended to VREF/2 ±200 mV,
increasing the acceptable voltage range in cases
where additional shift is required. These features help
avoid FDA output swing limitations while driving a fullscale ADC input signal, all without adding extra
components.
V+
AVDD
+
VS+
A fully-differential SAR ADC requires that the input
common-mode voltage remain constant throughout the
ADC full-scale range. This common-mode voltage
restriction is typically limited to one-half of the ADC
reference voltage (VREF/2) ±100 mV.
Two potential solutions to avoid output swing
limitations in the FDA stage include:
1. Generating a negative supply rail to drive the
negative FDA supply pin below ground. Applying a
negative supply rail (i.e. -200 mV) can allow the
amplifier output to swing all the way to 0 V without
changing the output common-mode voltage.
2. Shifting the VOCM voltage above mid-reference to
avoid the negative rail. Increasing the output
common-mode voltage also increases the linear
output swing, but most fully-differential SAR ADCs
only allow ±100 mV. This may not always be
sufficient to avoid the negative rail completely with
a full-scale input signal.
AIN_P
FDA
VOCM
±
VS-
Introduction
ADS9224R
AIN_N
REFby2
AVSS
VREF/2 (+100 mV optional)
Figure 2. ADS9224R REFby2 Driving FDA VOCM
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Adjusting the Input Common-Mode Voltage for SAR ADCs to Avoid Amplifier
Output Swing Limitations Ryan Andrews
Copyright © 2019, Texas Instruments Incorporated
1
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Measurement Comparison
Results
The performance benefit of the ADS9224R can be
directly measured using the ADS9224REVM-PDK.
This evaluation module features a wide-bandwidth
FDA (TI's THS4551) at the input of each ADS9224R
channel operating from the same 5-V supply. The
following measurements were collected while sampling
at the full 3-MSPS throughput, placing the highest
current demand on the THS4551 outputs. The
ADS9224R is set to use the internal 2.5-V reference
voltage with a gain of 1.634 V/V to achieve VREF =
4.096 V. The mid-reference output ("REFby2") is
connected to the VOCM pin of each THS4551 and
varied from 2.048 V to 2.148 V.
The impact on AC performance is analyzed by
computing an FFT on the data collected from each
channel. The clipped sine wave output from Channel 1
introduces significant harmonic distortion to the
frequency spectrum. The resulting measured SNR and
THD (63.13 dB and -59.69 dB, respectively) are well
below the data sheet specifications of the ADS9224R.
The results from Channel 2 are substantially improved,
as shifting the common-mode voltage greatly reduced
the harmonic content in the THS4551 output. As a
result, this system is able to meet the typical SNR and
THD specifications of the ADS9224R (93 dB and -109
dB, respectively).
CHANNEL 1
5V
VS+
AVDD
+
VOCM
THS4551
ADS9224R
VS-
±
AIN_P
AIN_N
REFby2
AVSS
2.048 V
CHANNEL 2
5V
VOCM
THS4551
±
VS-
VS+
AVDD
+
AIN_P
ADS9224R
AIN_N
REFby2
AVSS
2.148 V = 2.048 V + 100 mV
Figure 3. ADS9224R REFby2 Comparison Setup
Figure 5. FFT and AC Results for Ch1 and Ch2
The applied input signal to each channel is a 2-kHz
single-ended sine wave with an amplitude of 8.17 VPP
(approximately -0.023 dBFS). Observing the noninverting output from each THS4551 shows that the
negative peak of the Channel 1 output is clipped and
distorted, while the Channel 2 output remains nearly
60 mV away from the negative rail.
Conclusion
By shifting the input common-mode voltage, the
design met the data sheet performance criteria of the
ADS9224R with an 8.17-VPP input signal. The
amplitude must be limited to 8.015 VPP in order to
achieve the same performance without the shift. In
other words, shifting the input common-mode by 100
mV increased the linear differential output swing by
about 150 mVPP to utilize the entire ADC full-scale
range and maintain performance.
Table 1. Alternative Device Recommendations
Device
Description
ADS9234R
14-bit, 3.5-MSPS, Dual-Channel, Simultaneous-Sampling
SAR ADC
THS4552
Dual-Channel, Low-Noise, Precision, 150-MHz, FullyDifferential Amplifier
Figure 4. VOCM = 2.048 V (Ch1) vs. 2.148 V (Ch2)
2
Adjusting the Input Common-Mode Voltage for SAR ADCs to Avoid Amplifier
Output Swing Limitations Ryan Andrews
Copyright © 2019, Texas Instruments Incorporated
SBAA406 – September 2019
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