Datasheet
+
R
F
-
C
F
IF R
A
< < R
F
C
F
c =
¨
©
§
1 +
R
B
R
A
¨
©
§
R
A
C
F
c
R
B
OP AMP
OPEN LOOP
GAIN
NOISE GAIN WITH NO C
F
NOISE GAIN WITH C
F
f
Z
f
P
A
0
f
Z
=
1
2S
R
F
C
IN
f
P
=
A
0
2S
R
F
(C
IN
+C
F
)
GAIN
FREQUENCY
LMV796, LMV797
SNOSAU9D –MARCH 2006–REVISED MARCH 2013
www.ti.com
Figure 53. C
F
Selection for Stability
Calculating C
F
from Equation 3 can sometimes return unreasonably small values (<1 pF), especially for high
speed applications. In these cases, it is often more practical to use the circuit shown in Figure 54 in order to
allow more reasonable values. In this circuit, the capacitance C
F
′ is (1+ R
B
/R
A
) times the effective feedback
capacitance, C
F
. A larger capacitor can now be used in this circuit to obtain a smaller effective capacitance.
For example, if a C
F
of 0.5 pF is needed, while only a 5 pF capacitor is available, R
B
and R
A
can be selected
such that R
B
/R
A
= 9. This would convert a C
F
′ of 5 pF into a C
F
of 0.5 pF. This relationship holds as long as R
A
<< R
F
.
Figure 54. Obtaining Small C
F
from Large C
F
′
LMV796 AS A TRANSIMPEDANCE AMPLIFIER
The LMV796 was used in the designs for a number of amplifiers with varying transimpedance gains and source
capacitances. The gains, bandwidths and feedback capacitances of the circuits created are summarized in
Table 1. The frequency responses are presented in Figure 55 and Figure 56. The feedback capacitances are
slightly different from the formula in Equation 3, since the parasitic capacitance of the board and the feedback
resistor R
F
had to be accounted for.
Table 1.
Transimpedance, A
TI
C
IN
C
F
−3 dB Frequency
470000 50 pF 1.5 pF 350 kHz
470000 100 pF 2.0 pF 250 kHz
470000 200 pF 3.0 pF 150 kHz
47000 50 pF 4.5 pF 1.5 MHz
47000 100 pF 6.0 pF 1 MHz
47000 200 pF 9.0 pF 700 kHz
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