ADS8331_1007 Datasheet, PDF (19/43 Pages) Texas Instruments – Low-Power, 16-Bit, 500kSPS, 4-/8-Channel Unipolar Input ANALOG-TO-DIGITAL CONVERTERS with Serial Interface

ADS8331
ADS8332
www.ti.com
SBAS363 – DECEMBER 2009
Driver Amplifier Choice
In order to take advantage of the high sample rate offered by the ADS8331/32, the analog inputs to the converter
should be driven with low-noise operational amplifiers (op amps), such as the OPA365, OPA211, OPA827, or
THS4031. An RC filter is recommended at each of the input channels to low-pass filter noise generated by the
input driving sources. These channels can accept unipolar signals with voltages between IN_X and COM in the
range of 0V to V_REF. If RC filters are not used between the op amps and the input channels, the minimum –3dB
bandwidth required by the driving op amps for the sampled signals to settle to within 1/2 LSB of the final voltage
can be calculated using Equation 1:
f_-3dB ³
(n + 1) ´ ln(2)
2p ´ t_{SAMPLE_MIN}
(1)
Where:
n = resolution of the converter (n = 16 for the ADS8331/32).
t_{SAMPLE_MIN} = minimum acquisition time.
The minimum value of t_SAMPLE in the Electrical Characteristics tables is 238ns (3 CCLKs with the internal
oscillator at 12.6MHz). Substituting these values for n and t_{SAMPLE_MIN} into Equation 1 shows f_–3dB must be at
least 7.9MHz. This bandwidth can be relaxed if the acquisition time is increased or an RC filter is added between
the driving op amp and the corresponding input channel (refer to Texas Instruments' Application Report
SBAA173 and associated references for additional information, available for download at www.ti.com). The
OPA365 used in the source-follower (unity-gain) configuration is shown in Figure 35 with recommended values
for the RC filter.
Input Signal
(0V to 4V)
OPA365
20W
1000pF
MUXOUT ADCIN
5V
VA
IN_X
ADS8331
ADS8332
COM
Figure 35. Unipolar Input Drive Configuration
Bipolar to Unipolar Driver
In systems where the input signal is bipolar, op amps such as the OPA365 and OPA211 can be used in the
inverting configuration with a dc bias applied to the noninverting input in order to keep the input signal to the
ADS8331/32 within its rated operating voltage range. This configuration is also recommended when the
ADS8331/32 is used in signal-processing applications where good SNR and THD performance is required. The
dc bias can be derived from low-noise reference voltage ICs such as the REF5025 or REF5040. The input
configuration shown in Figure 36 is capable of delivering better than 91dB SNR and –99dB THD at an input
frequency of 1kHz. If bandpass filters are used to filter the input to the driving op amp, the signal swing at the
input of the bandpass filter should be small enough to minimize the distortion introduced by the filter. In these
cases, the gain of the circuit shown in Figure 36 can be increased to maintain a large enough input signal to the
ADS8331/32 to keep the system SNR as high as possible.
2.048VDC
600W
Input Signal
(-2V to +2V)
OPA211
600W
20W
1000pF
MUXOUT ADCIN
5V
VA
IN_X
ADS8331
ADS8332
COM
Figure 36. Bipolar Input Drive Configuration
Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s): ADS8331 ADS8332
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