Signal Conditioning
2.1 Signal Conditioning
The ADS8402/ADS8412EVM comes installed with the unity gain buffer (U2) wired for single-ended in to differential out configuration. The common-mode voltage is derived from a REF3040 reference IC and is adjustable using a potentiometer (R9). The common-mode voltage pin of the THS4503 is set to 2 V on the evaluation module. A single-ended input signal can be applied at pin connector P1 or via SMA connectors J2 (noninverting input). The buffer circuit can be reconfigured for a unipolar differential input by installing resistor R6 and R8 and removing R1. The inverting leg of the differential signal should be applied to either connector P1 pin 1 or SMA connector J4 (inverting input). See Table 2- 1 for the pinout of the analog connector, P1. See Chapter 6 for the EVM schematic.
Table 2- 1. Analog Input Connector
Description | Signal Name | Connector.Pin# | Signal Name | Description |
| | | | | |
Inverting input | (-) | P1.1 | P1.2 | + | Noninverting input |
Reserved | N/A | P1.3 | P1.4 | N/A | Reserved |
Reserved | N/A | P1.5 | P1.6 | N/A | Reserved |
Reserved | N/A | P1.7 | P1.8 | N/A | Reserved |
Pin tied to ground | AGND | P1.9 | P.10 | N/A | Reserved |
Pin tied to ground | AGND | P1.11 | P1.12 | N/A | Reserved |
Reserved | N/A | P1.13 | P1.14 | N/A | Reserved |
Pin tied to ground | AGND | P1.15 | P1.16 | N/A | Reserved |
Pin tied to ground | AGND | P1.17 | P1.18 | N/A | Reserved |
Reserved | N/A | P1.19 | P1.20 | REF+ | External reference input |
It is recommended the analog input to any SAR-type converter be buffered. The amplifier circuit in Figure 2- 1 is the buffer circuit used on the ADS8402/ADS8412EVM. This circuit consists of the THS4503, a high-speed fully differential amplifier configured as a single-ended in to differential out, unity gain buffer. The circuit shown in Figure 2- 1 was optimized to achieve the ac (i.e., SNR, THD, SFDR, etc.) specifications listed in the ADS8402 and ADS8412 data sheets. The 60-pF and 6800-pF capacitors in the signal path are polypropylene type, manufactured by the WIMA Corporation. Polypropylene capacitors cause the least distortion of the input signal.
