Datasheet

2SR
F
C
T
Where, f
Z
1
#
and f
P
=
2SR
F
C
F
1
NG =
1 + sR
F
(C
T
+ C
F
)
1 + sC
F
R
F
+
-
C
PD
C
IN
LMH6618
R
F
C
F
V
S
+
-
LMH6618
1 k:
51:
51:
0.1:
0A to 1A
1 k:
+5V
LMH6618, LMH6619
www.ti.com SNOSAV7E –AUGUST 2007–REVISED OCTOBER 2012
Figure 70. Current Sense Amplifier
TRANSIMPEDANCE AMPLIFIER
By definition, a photodiode produces either a current or voltage output from exposure to a light source. A
Transimpedance Amplifier (TIA) is utilized to convert this low-level current to a usable voltage signal. The TIA
often will need to be compensated to insure proper operation.
Figure 71. Photodiode Modeled with Capacitance Elements
Figure 71 shows the LMH6618 modeled with photodiode and the internal op amp capacitances. The LMH6618
allows circuit operation of a low intensity light due to its low input bias current by using larger values of gain (R
F
).
The total capacitance (C
T
) on the inverting terminal of the op amp includes the photodiode capacitance (C
PD
) and
the input capacitance of the op amp (C
IN
). This total capacitance (C
T
) plays an important role in the stability of
the circuit. The noise gain of this circuit determines the stability and is defined by:
(1)
(2)
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