Datasheet
+
-
LMP7721
I
bias
R
S
V
+
V
-
V
S
V
in
+
-
V
in
= V
S
± (I
bias
x R
S
)
Error
+
-
LMP7721
LIGHT
R
F
C
F
+
-
10 k:
10 k:
V
REF
V
out
100:
V
+
V
-
0.1 µF
0.1 µF
0.1 µF
0.1 µF
TRIAX
LMP7715
LMP7721
SNOSAW6D –JANUARY 2008–REVISED MARCH 2013
www.ti.com
Figure 49. LMP7721 as Transimpedance Amplifier
The current generated by the photodiode is fed to the signal conductor of the triax and then sent to the inverting
input of the LMP7721. The LMP7721’s non-inverting input is biased at V
REF
/2 for level shifting purposes. In this
application, the non-inverting input is a low impedance node and hence is used to drive the LMP7715 which acts
as a guard driver. The output of the guard driver is connected to the guard of the triax via a 100Ω isolation
resistor. Ideally, the inverting and the non-inverting inputs of the amplifier are kept at the same potential through
the operation of the amplifier. By connecting the signal conductor to the inverting input and letting the non-
inverting input drive the guard, the signal conductor and the guard are kept at the same potential which prevents
leakage from the signal source.
pH ELECTRODE AMPLIFIER EXAMPLE (NON-INVERTING CONFIGURATION)
The output of a pH electrode ranges from 415 mV to −415 mV as the pH changes from 0 to 14 at 25°C. The
output impedance of a pH electrode is extremely high, ranging from 10 MΩ to 1000 MΩ. The ultra low input bias
current of the LMP7721 allows the voltage error produced by the input bias current and electrode resistance to
be minimal. For example, the output impedance of the pH electrode used is 10 MΩ, if an op amp with 3 nA of
I
bias
is used, the error caused due to this amplifier’s input bias current and the source resistance of the pH
electrode is 30 mV! This error can be greatly reduced to 30 nV by using the LMP7721.
Figure 50. Error Caused by Amplifier’s Input Bias Current and Sensor Source Impedance
Figure 51 is an example of the LMP7721 used as a pH sensor amplifier.
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