ADS8331IPWR datasheet(19/43 Pages) TI

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3dB

(n+1)

ln(2)

p ´

SAMPLE_MIN

ADS8331

ADS8332

COM

OPA365

MUXOUT

ADCIN

InputSignal

(0Vto4V)

20W

1000pF

ADS8331

ADS8332

COM

OPA211

MUXOUT

ADCIN

2.048VDC

600W

InputSignal

( 2Vto+2V)

20W

1000pF

600W

ADS8331

ADS8332

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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 INX and COM in the

range of 0V to VREF. 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:

(1)

Where:

n = resolution of the converter (n = 16 for the ADS8331/32).

tSAMPLE_MIN = minimum acquisition time.

The minimum value of tSAMPLE in the Electrical Characteristics tables is 238ns (3 CCLKs with the internal

oscillator at 12.6MHz). Substituting these values for n and tSAMPLE_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.

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.

Figure 36. Bipolar Input Drive Configuration

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