LMH6642_16 Datasheet, PDF (22/39 Pages) Texas Instruments – LMH664x Low Power, 130 MHz, 75 mA Rail-to-Rail Output Amplifiers

LMH6642, LMH6643, LMH6644
SNOS966Q – MAY 2001 – REVISED SEPTEMBER 2014
9 Application and Implementation
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NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
This device family was designed to avoid output phase reversal. With input overdrive, the output is kept near
supply rail (or as closed to it as mandated by the closed loop gain setting and the input voltage). See Figure 56.
However, if the input voltage range of −0.5 V to 1 V from V⁺ is exceeded by more than a diode drop, the internal
ESD protection diodes will start to conduct. The current in the diodes should be kept at or below 10 mA.
Output overdrive recovery time is less than 100 ns as can be seen in Figure 57.
9.2 Typical Application
C_f
5pF
V_bias
Photodiode
Equivalent
R_f
Circuit
1k:
R_bias
C1
100nF
Q1
2N3904
-1mA_PP
V_CC =
+5V
-
Photodiode
I_d
C_d
10
-
200pF
R_d
×^100k:
R5
x
510:
R2
1.8k:
+
V_out
R11
D1 910
R10
1N4148 :
1k:
R3
1k:
+5V
Figure 55. Single Supply Photodiode I-V Converter
9.2.1 Design Requirements
The circuit shown in Figure 55 is used to amplify the current from a photodiode into a voltage output. In this
circuit, the emphasis is on achieving high bandwidth and the transimpedance gain setting is kept relatively low.
Because of its high slew rate limit and high speed, the LMH664X family lends itself well to such an application.
This circuit achieves approximately 1V/mA of transimpedance gain and capable of handling up to 1mApp from
the photodiode. Q1, in a common base configuration, isolates the high capacitance of the photodiode (Cd) from
the Op Amp input in order to maximize speed. Input is AC coupled through C1 to ease biasing and allow single
supply operation. With 5-V single supply, the device input/output is shifted to near half supply using a voltage
divider from VCC. Note that Q1 collector does not have any voltage swing and the Miller effect is minimized. D1,
tied to Q1 base, is for temperature compensation of Q1’s bias point. Q1 collector current was set to be large
enough to handle the peak-to-peak photodiode excitation and not too large to shift the U1 output too far from
mid-supply.
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