Texas Instruments ADS8881EVM-PDK, Руководство пользователя

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AINP
AINN
10000pF
10
10
-
+
+
-
10
10
1k
1k
1k
THS4521 Vocm =+2.25V
-2.15V
+2.15V
1k
0V
-2.15V
+2.15V
0V
0.1V
4.3V
2.25V
0.1V
4.3V
2.25V
-4.3V
+4.3V
0V
5V
A0(-)
A0(+)
EVM Analog Interface
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2 EVM Analog Interface
The ADS8881EVM is designed for easy interfacing to multiple analog sources. SMA connectors allow
the EVM to have input signals connected through coaxial cables. In addition, the Samtec connector
provides a convenient 10-pin, dual-row, header/socket combination at J1. Consult Samtec at
http://www.samtec.com or call 1-800-SAMTEC-9 for a variety of mating connector options. All analog
inputs are buffered by THS4521 high-speed fully differential amplifier in order to properly drive the
ADS8881 ADC inputs.
Use appropriate caution when handling these pins. Table 1 summarizes the pinout for analog interface J1.
Table 1. J1: Analog Interface Pinout (Samtec 10 x 2)
Pin Number Signal Description
CH0 negative differential input. This pin can be
J1.10 A0(-)
grounded for single ended signals.
CH0 positive differential input or input for single
J1.12 A0(+)
ended signals
Table 2 lists the SMA analog inputs.
Table 2. SMA Analog Inputs
Pin Number Signal Description
CH0 negative differential input. This pin can be
J4 A0(-)
grounded for single ended signals.
CH0 positive differential input or input for single
J5 A0(+)
ended signals
2.1 Differential Input Signal Configuration
The ADS8881 can only convert differential signals with common mode between 0 V and V
REF
, but the
THS4521 can condition a bipolar signal changing its common mode. This EVM sets the THS4521 output
common mode to 2.25 V, that corresponds to V
REF
/ 2. Thus, a bipolar differential signal with a common
mode of 0 V can be applied at the EVM inputs A0(-) and A0(+). Then, the THS4521 shifts the common
mode to 2.25 V. Although input common mode can be corrected, since the THS4521 is powered by a 5-V
supply, the input signals must limit the differential range from –4.3 V to 4.3 V to avoid saturating the
amplifier output.
Figure 1 illustrates an input signal with 0-V common mode and 8.6-V differential, which
common mode is changed from 0 V to 2.25 V.
Figure 1. Differential Input Example
4 ADS8881EVM-PDK SBAU211 – June 2013
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