ADS8328IBPW datasheet(22/50 Pages) TI

22 / 50 page

Device in Hold Mode

AGND

150 W

+IN

−IN

AGND

+VA

150 W

4 pF

40 pF

ADS8327

+IN

THS4031

20 W

470pF

50 W

InputSignal

(0Vto4V)

+VA

ADS8327

ADS8328

SLAS415E

– APRIL 2006 – REVISED JANUARY 2011

www.ti.com

Figure 51. Input Equivalent Circuit

Driver Amplifier Choice

The analog input to the converter needs to be driven with a low noise, op-amp like the THS4031 or

OPA365

. An RC filter is recommended at the input pins to low-pass filter the noise from the source. Two

resistors of 20

Ω and a capacitor of 470 pF is recommended. The input to the converter is a unipolar input voltage

in the range 0 V to VREF. The minimum -3dB bandwidth of the driving operational amplifier can be calculated to:

f3db = (ln(2) ×(n+1))/(2p × tACQ)

where n is equal to 16, the resolution of the ADC (in the case of the ADS8327/28). When tACQ = 238 ns

(minimum acquisition time), the minimum bandwidth of the driving amplifier is 7.9 MHz. The bandwidth can be

relaxed if the acquisition time is increased by the application. The OPA365, OPA827, or THS4031 from Texas

Instruments are recommended. The THS4031 used in the source follower configuration to drive the converter is

shown in the typical input drive configuration, Figure 52. For the ADS8330, a series resistor of 0

Ω should be

used on the COM input (or no resistor at all).

Bipolar to Unipolar Driver

In systems where the input is bipolar, the THS4031 can be used in the inverting configuration with an additional

DC bias applied to its + input so as to keep the input to the ADS8327/28 within its rated operating voltage range.

This configuration is also recommended when the ADS8327/28 is used in signal processing applications where

good SNR and THD performance is required. The DC bias can be derived from the REF3225 or the REF3240

reference voltage ICs. The input configuration shown in Figure 53 is capable of delivering better than 91-dB SNR

and

–96-dB THD at an input frequency of 10 kHz. In case bandpass filters are used to filter the input, care

should be taken to ensure that the signal swing at the input of the bandpass filter is small so as to keep the

distortion introduced by the filter minimal. In such cases, the gain of the circuit shown in Figure 53 can be

increased to keep the input to the ADS8327/28 large to keep the SNR of the system high. Note that the gain of

the system from the + input to the output of the THS4031 in such a configuration is a function of the gain of the

AC signal. A resistor divider can be used to scale the output of the REF3225 or REF3240 to reduce the voltage

at the DC input to THS4031 to keep the voltage at the input of the converter within its rated operating range.

Figure 52. Unipolar Input Drive Configuration

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ADS8328IBPW Datasheet(PDF) 22 Page - Texas Instruments

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