Texas Instruments ADS9120, User Manual, Page: 7 / 38

2.5 V
Q
ADC_V
CM
Q
2.6 V
ADC_ V
CM
=
5 ×
tr
•
À
tr
•
À
+ (R
1
//
tr
•
À
)
×
l
1 +
s
•
À
s
•
À
+ R
S
p F l
2.5 ×
s
•
À
s
•
À
+ R
S
p
ADC_ V
CM
=
5 × ( sr • À // tr • À )
( sr • À // tr • À ) + (R
1
// tr • À )
×
l
1 +
s • À
s • À + R
S
p
20k Ÿ
280k Ÿ
5.0V
20k Ÿ
10k Ÿ
OPA625_CM
OPA376
Gain = 1
J2
J1
R1
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Analog Interface
7 SBAU262 – August 2016
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ADS9120EVM-PDK
Figure 2. Common-Mode Selection Jumpers
Table 3. J1 and J2 Configuration per Input Common-Mode
J1 Setting
(R1 Comp)
J2 Setting
External Signal
Common-Mode
Differential Source Type
CLOSED CLOSED 0 V Bipolar: If R1 = 280 k Ω , R
S
range is 0 Ω to 32 Ω
CLOSED OPEN 2.5 V Unipolar: must change R1 to match R
S
CLOSED OPEN Floating AC-coupled bipolar: If R1 = 280 k
Ω
, no R
S
restriction
2.2.3 R1 Setting vs Source Impedance
The external source impedance (R
S
) adds up to the 1 k Ω of the input resistor, thereby moving the output
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 k Ω that allows an external source impedance (R
S
) range between
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 k
Ω
for any given source impedance. The ADC common-mode for 0-V input common-mode setting is
calculated using Equation 1 .
(1)
In the case of unipolar input signals with a 2.5-V common-mode, the ADC common-mode is calculated
using
Equation 2 .
(2)
For Equation 1 and Equation 2 the value of R1 must be calculated to satisfy Equation 3 :
(3)