Datasheet

Driver Amplifier Choice
Bipolar to Unipolar Driver
ADS7279
+IN
-IN
THS4031
20W
20W
470pF
50W
Input
Signal
(0Vto4V)
5V
+VA
ADS7279
+IN
-IN
THS4031
20W
20W
1VDC
Input
Signal
(-2Vto2V)
5V
+VA
470pF
ADS7279
ADS7280
SBAS436A – MAY 2008 – REVISED JUNE 2009 ............................................................................................................................................................... www.ti.com
The analog input to the converter must be driven with a low-noise operational amplifier such as the THS4031 or
OPA365 . An RC filter is recommended at the input pins to low-pass filter the noise from the source. Two 20 Ω
resistors and a 470pF capacitor are recommended. The input to the converter is a unipolar input voltage in the
range of 0V to V
REF
. The minimum – 3dB bandwidth of the driving operational amplifier can be calculated as:
f
3db
= (ln(2) × (n+1))/(2 π × t
ACQ
)
where n is equal to 14, the resolution of the ADC (in the case of the ADS7279/80). When t
ACQ
= 120ns (minimum
acquisition time), the minimum bandwidth of the driving amplifier is 13.8MHz. The bandwidth can be relaxed if
the acquisition time is increased by the application. Figure 54 shows the THS4031 used in the source follower
configuration to drive the converter in a typical input drive configuration. For the ADS7280, a series resistor of 0 Ω
should be used on the COM input (or no resistor at all).
In systems where the input is bipolar, the THS4031 can be used in an inverting configuration with an additional
dc bias applied to its positive input to keep the input to the ADS7279/80 within the rated operating voltage range.
This configuration is also recommended when the ADS7279/80 is used in signal processing applications where
good SNR and THD performance are required. The dc bias can be derived from the REF5025 or the REF5040
reference voltage ICs. The input configuration shown in Figure 55 is capable of delivering better than 85dB SNR
and – 100dB THD at an input frequency of 10kHz. If 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, in order to keep the distortion
introduced by the filter minimal. In this case, the gain of the circuit shown in Figure 55 can be increased to keep
the input to the ADS7279/80 large in order to maintain a high SNR of the system. Note that the gain of the
system from the positive input to the output of the THS4031 in such a configuration is a function of the ac signal
gain. A resistor divider can be used to scale the output of the REF5025 or REF5040 to reduce the voltage at the
dc input to the THS4031 to maintain the voltage at the converter input within its rated operating range.
Figure 54. Unipolar Input Drive Configuration
Figure 55. Bipolar Input Drive Configuration
24 Submit Documentation Feedback Copyright © 2008 – 2009, Texas Instruments Incorporated
Product Folder Link(s): ADS7279 ADS7280