Texas Instruments ADS8861EVM-PDK, Руководство пользователя, Страница: 5 / 21

5V
5V
0.22
+
-
+
1k
1µF
+
-
+
OPA333
THS4281
10µF
1µF
20k
1k
1µF
Vref
4.5V from
REF5045
AINP
AINN
10000pF
10
10
-
+
+
-
10
10
1k
1k
1k
THS4521
Vocm =+2.25V
1k
-4.3V
+4.3V
0V
0.1V
4.4V
2.25V
0.1V
4.4V
2.25V
-4.3V
+4.3V
0V
5V
A0(+)
A0(-)
0V
+8.6V
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EVM Analog Interface
2.2 Single-Ended Signal Configuration
The ADS8861 can only convert single-ended signals between 0 V and V
REF
. The THS4521 can condition a
single-ended signal to a differential signal, thus allowing a larger input voltage range for the EVM input
A0(+). The single-ended signal range can be 0 V to 8.6 V or –4.3 V to 4.3 V to avoid saturating the
amplifier output. The EVM A0(–) input should be grounded for single-ended signals, as shown in
Figure 2 .
Figure 2. Single-Ended Signal Example
2.3 Voltage Reference
The reference voltage should be below 5 V because the EVM is powered by a 5-V analog supply. This
EVM uses a 4.5 V value created by the onboard REF5045. This value is then filtered by an RC filter with a
160-Hz cutoff frequency to minimize noise contribution. Finally, the filtered reference value is buffered by
the THS4281 that functions as the driver for the ADC reference input pin. To meet performance
specifications, a 10-
μ F capacitor with a small ESR is also present at the ADC reference input pin. This
large capacitive load reduces the THS4281 effective bandwidth to 2 MHz, making the THS4281 total noise
contribution approximately 22
μ
VRMS
. The OPA333 and the additional feedback are optional, but do
complement the THS4281 by minimizing offset and drift.
Figure 3. THS4281 Reference Driver with Complementary OPA333 for Drift and Offset Correction
5 SBAU214 – October 2013 ADS8861EVM-PDK
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