270000
1000
10
8
.
2
177500
1000
10
9
.
1
8
1
8
u
u
S
S
R
R
R
9000
67
10
7
.
2
9000
96
10
6
.
2
7
1
7
u
u
S
S
R
R
R
Analog Interface
Figure 2. Common Mode Selection Jumpers
Table 3. J1 and J2 Configuration per Input Common Mode
J1 Setting
External Signal
J2 Setting
Differential Source Type
(R
1
Comp)
Common Mode
CLOSED
CLOSED
0 V
Bipolar: If R1 = 280 k
Ω
, R
S
range is 0
Ω
to 32
Ω
CLOSED
OPEN
2.25 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
R
1
Setting vs Source Impedance
The external source impedance ( R
S
) will add up to the 1 k
Ω
of the input resistor, thereby moving the
output common mode of the OPA625 amplifiers. To compensate for this, R
1
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 R
1
as 280 k
Ω
that allows an external source impedance ( R
S
) range between 0
Ω
to 32
Ω
for 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 R
1
should remain at 280
k
Ω
for any given source impedance. The range of values of R
1
for 0-V common mode is determined using
.
(1)
In the case of unipolar input signals, R
1
must be replaced since 280 k
Ω
is not large enough to compensate
for any practical value of external source impedance (R
S
). The range of values of R
1
for 2.25-V common
mode is determined using
.
(2)
7
SBAU249 – October 2015
ADS9110EVM-PDK
Copyright © 2015, Texas Instruments Incorporated