SBAU439 January 2024
2.1.1.2 Precision, High-CMRR Circuit: PGA855
Figure 3-3 shows the schematic for the PGA855 input circuit connected to ADC B. The PGA855 is a high-bandwidth programmable gain instrumentation amplifier with fully differential outputs. This circuit is designed for input signal frequencies up to approximately 300kHz. The gain of the PGA855 is set by the combination of three binary gain pins, which are configured using the corresponding jumpers on the EVM (see Table 3-1).
The PGA855 uses separate supply voltages for the input stage and the output stage. The input stage supply rails (VS+ and VS-) can range up to ±18 V and must be provided externally. Connect the VS+ and VS- supply voltages and a ground wire from an external power source to the J3 terminal block on the EVM. The output stage supply rails (LVDD and LVSS) are shared with the ADS9227 analog supplies (AVDD_5V and GND). This is done intentionally to limit the output swing of the PGA855 and to protect the ADS9227 inputs from overdrive.
| A2 (JP8) | A1 (JP9) | A0 (JP10) | Gain (V/V) |
|---|---|---|---|
|
0 |
0 |
0 |
0.125 |
|
0 |
0 |
1 |
0.25 |
|
0 |
1 |
0 |
0.5 |
|
0 |
1 |
1 |
1 |
|
1 |
0 |
0 |
2 |
|
1 |
0 |
1 |
4 |
|
1 |
1 |
0 |
8 |
|
1 |
1 |
1 |
16 |
Figure 2-3 PGA855, High-CMRR Circuit for
ADC B