Texas Instruments ADS7854EVM-PDK, Руководство пользователя, Страница: 5 / 35

AINP
AINN
820 pF
10
+
1 k
THS4521
Vocm = +FSR_ADC / 2 = 2.5 V
-2.15 V
+2.15 V
1 k
0 V
0.35 V
4.65 V
2.5 V
0.35 V
4.65 V
2.5 V
Å
4.3 V
+4.3 V
0 V
5 V
A0(
Å
)
A0(+)
FSR_ADC = ±2 × V
REF
= ±5 V
1 k
1 k
10
10
10
Å
2.15 V
+2.15 V
0 V
+
Å
Å
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EVM Analog Interface
2.1 Analog Input Range Settings
The ADSxx54 dual simultaneous ADC supports fully-differential analog input signals on both channels.
The full-scale range (FSR) of the ADSxx54 device can be programmed to either a (±1 × V
ref
) or (±2 × V
ref
)
range by setting bit B9 of the ADSxx54 configuration register. This register is common to both ADCs in the
device (ADC_A and ADC_B). Configure the full-scale range setting in the EVM by navigating to the
ADSxx54EVM Settings page on the graphical user interface (GUI). Refer to Section 6.3 of this user guide
for more information. Set the ADC differential inputs at a common-mode voltage of FSR_ADC_A / 2 and
FSR_ADC_B / 2, respectively. Configure jumpers JP5 and JP6 on the ADSxx54EVM to bias the ADC
input signal at the correct common-mode voltage level.
Table 3 describes the appropriate range jumper
settings.
Table 3. Analog Input Range Jumper Settings
Jumper
Number Default Position Description
Installed when FSR = ±1 × V
ref
; removed when FSR = ±2 × V
ref
.
JP5 Installed
Sets ADC_A common-mode voltage.
Installed when FSR = ±1 × V
ref
; removed when FSR = ±2 × V
ref
.
JP6 Installed
Sets ADC_B common-mode voltage.
The ADSxx54EVM incorporates two THS4521 fully-differential amplifiers to drive the ADC inputs. The
fully-differential amplifiers shift the signal to the appropriate common-mode voltage level.
Figure 2 shows
an example where the ADSxx54 device is configured with the ±2 × V
ref
FSR. A differential input signal with
a common-mode voltage of 0 V is applied to the inputs of the THS4521. The THS4521 shifts the signal to
the required common-mode voltage of FSR / 2. Because the THS4521 is powered by a 5-V supply , the
input signals must be limited to a differential voltage from –4.3 V to 4.3 V to avoid saturating the amplifier
output.
Figure 2. THS4521 Fully-Differential Amplifier Driver
5 SBAU209 – April 2014 ADS8354EVM-PDK and ADS7854EVM-PDK
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