Datasheet

Device in Hold Mode
AGND
150 W
+IN
−IN
AGND
+VA
150 W
4 pF
4 pF
40 pF
40 pF
ADS8327
+IN
-IN
THS4031
20 W
20 W
470pF
50 W
InputSignal
(0Vto4V)
5V
+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 20Ω and a capacitor of 470 pF is recommended. The input to the converter is a unipolar input voltage
in the range 0 V to V
REF
. The minimum -3dB bandwidth of the driving operational amplifier can be calculated to:
f
3db
= (ln(2) ×(n+1))/(2p × t
ACQ
)
where n is equal to 16, the resolution of the ADC (in the case of the ADS8327/28). When t
ACQ
= 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 0Ω 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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