Datasheet

+
R
F
-
C
F
IF R
A
< < R
F
C
F
c =
¨
©
§
1 +
R
B
R
A
¨
©
§
R
A
C
F
R
B
C
F
=
C
IN
GBWP  2 SR
F
C
CM
I
IN
R
F
V
OUT
+
-
+
-
V
B
C
F
C
D
V
OUT
I
IN
- R
F
=
C
IN
= C
D
+ C
CM
LMP7715, LMP7716, LMP7716Q
www.ti.com SNOSAV0E –MARCH 2006–REVISED MARCH 2013
Figure 54. Transimpedance Amplifier
A feedback capacitance C
F
is usually added in parallel with R
F
to maintain circuit stability and to control the
frequency response. To achieve a maximally flat, 2
nd
order response, R
F
and C
F
should be chosen by using
Equation 3
(3)
Calculating C
F
from Equation 3 can sometimes result in capacitor values which are less than 2 pF. This is
especially the case for high speed applications. In these instances, it is often more practical to use the circuit
shown in Figure 55 in order to allow more sensible choices for C
F
. The new feedback capacitor, C
F
′, is (1+
R
B
/R
A
) C
F
. This relationship holds as long as R
A
<< R
F
.
Figure 55. Modified Transimpedance Amplifier
SENSOR INTERFACE
The LMP7715/LMP7716/LMP7716Q have low input bias current and low input referred noise, which make them
ideal choices for sensor interfaces such as thermopiles, Infra Red (IR) thermometry, thermocouple amplifiers,
and pH electrode buffers.
Thermopiles generate voltage in response to receiving radiation. These voltages are often only a few microvolts.
As a result, the operational amplifier used for this application needs to have low offset voltage, low input voltage
noise, and low input bias current. Figure 56 shows a thermopile application where the sensor detects radiation
from a distance and generates a voltage that is proportional to the intensity of the radiation. The two resistors, R
A
and R
B
, are selected to provide high gain to amplify this signal, while C
F
removes the high frequency noise.
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