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Signal Conditioning
2-2
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.