Common-Mode Selection Jumpers - Texas Instruments ADS8910BEVM-PDK User Manual

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J1 Setting
J2 Setting
(R1 Comp)
CLOSED CLOSED
CLOSED OPEN
CLOSED OPEN
2.2.3 R1 Setting vs Source Impedance
The external source impedance (R
common-mode of the OPA625 amplifiers. To compensate for this change in output common-mode, R1
can be modified according to the particular external source impedance value used with the evaluation
board to allow full-scale input range without saturating the OPA625 amplifiers.
The board is shipped with R1 as 280 kΩ that allows an external source impedance (R
0 Ω to 32 Ω for a 0-V common-mode configuration (J1:closed and J2:closed). For floating or ac-coupled
signals, the input common-mode is set by the OPA625 amplifiers themselves and R1 must remain at 280
kΩ for any given source impedance. The ADC common-mode for 0-V input common-mode setting is
calculated using Equation
ADC_ V =
CM
(sr•À//tr•À) + (R
In the case of unipolar input signals with a 2.5-V common-mode, the ADC common-mode is calculated
using Equation 2.
ADC_ V =
CM
tr•À + (R
For Equation 1 and
2.5 V Q ADC_V
SBAU268 – July 2016
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OPA625_CM
OPA376
Gain = 1
10kŸ
J2
Figure 2. Common-Mode Selection Jumpers
Table 3. J1 and J2 Configuration per Input Common Mode
External Signal
Common-Mode
0 V
2.5 V
Floating
) adds up to the 1 kΩ of the input resistor, thereby moving the output
S
1.
5 × (sr•À//tr•À)
//tr•À)
1
5 × tr•À
× l1 +
// tr•À)
1
Equation 2 the value of R1 must be calculated to satisfy
Q 2.6 V
CM
Copyright © 2016, Texas Instruments Incorporated
J1
R1
280kŸ
20kŸ
20kŸ
Differential Source Type
Bipolar: If R1 = 280 kΩ, R
Unipolar: must change R1 to match R
AC-coupled bipolar: If R1 = 280 kΩ, no R
s•À
× l 1 + p
s•À + R
S
s•À 2.5 × s•À
p F l p
s•À + R s•À + R
S S
Analog Interface
5.0V
range is 0 Ω to 32 Ω
S
S
restriction
S
) range between
S
Equation 3:
ADS8910BEVM-PDK
(1)
(2)
(3)
7