Texas Instruments ADS8861EVM-PDK, Руководство пользователя, Страница: 4 / 21

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
4.4V
2.25V
0.1V
4.4V
2.25V
-4.3V
+4.3V
0V
5V
A0(-)
A0(+)
EVM Analog Interface
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2 EVM Analog Interface
The ADS8861EVM 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 and socket combination at J1. Consult Samtec at
www.samtec.com or call 1-800-SAMTEC-9 for a variety of mating connector options. All analog inputs are
buffered by the THS4521 high-speed, fully-differential amplifier in order to properly drive the ADS8861
ADC inputs.
Use appropriate caution when handling these pins. Table 1 summarizes the pinout for the analog interface
J1.
Table 1. J1: Analog Interface Pinout (Samtec 10 x 2)
Pin Number Signal Description
Channel 0 negative differential input. This pin can be grounded for single-
J1.10 A0(–)
ended signals.
J1.12 A0(+) Channel 0 positive differential input or input for single-ended signals.
Table 2 lists the SMA analog inputs.
Table 2. SMA Analog Inputs
Pin Number Signal Description
Channel 0 negative differential input. This pin can be grounded for single-
J4 A0(–)
ended signals.
J5 A0(+) Channel 0 positive differential input or input for single-ended signals.
2.1 Differential Input Signal Configuration
Although the ADS8861 can only convert unipolar differential signals, bipolar differential signals can be
applied at the EVM inputs because the EVM inputs are conditioned by the THS4521 fully-differential
amplifier. This EVM sets the THS4521 output common-mode voltage to 2.25 V, corresponding to V
REF
/ 2.
Thus, a bipolar differential signal with a common-mode voltage of 0 V can be applied at the EVM inputs
A0(–) and A0(+). Then, the THS4521 shifts the common-mode voltage to 2.25 V. Even though the input
common-mode voltage can be corrected, the input signals must limit the differential range from –4.3 V to
4.3 V to avoid saturating the amplifier output because the THS4521 is powered by a 5-V supply.
Figure 1
shows an input signal with a 0-V common-mode voltage and an 8.6-V differential voltage, where the
common-mode voltage is changed from 0 V to 2.25 V.
Figure 1. Differential Input Example
4 ADS8861EVM-PDK SBAU214 – October 2013
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