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1 signal conditioning – Texas Instruments ADS8402 User Manual

Page 12

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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.

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