LMH6612MA Datasheet, PDF(28/32 Page) Texas Instruments – Single Supply 345 MHz Rail-to-Rail Output Amplifiers

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LMH6612MA Datasheet, PDF (28/32 Pages) Texas Instruments – Single Supply 345 MHz Rail-to-Rail Output Amplifiers

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CURRENT SENSE AMPLIFIER AND OPTIMIZING

ACCURACY IN PRECESION APPLICATIONS

With itâs rail-to-rail output capability, low VOS, and low IB the

LMH6611 is an ideal choice for a current sense amplifier ap-

plication. Figure 11 shows the schematic of the LMH6611 set

up in a low-side sense configuration which provides a con-

version gain of 2V/A. Voltage error due to VOS can be calcu-

lated to be VOS x (1 + RF/RG) or 0.6 mV x 21 = 12.6 mV.

Voltage error due to IO is IO x RF or 0.5 ÂµA x 1 kâ¦ = 0.5 mV.

Hence worst case total voltage error is 12.6 mV + 0.5 mV or

13.1 mV which translates into a current error of 13.1 mV/(2 V/

A) = 6.55 mA.

This circuit employs DC source resistance matching at the

two input terminals in order to minimize the output DC error

caused by input bias current. Another technique to reduce

output offset in a non-inverting amplifier configuration is to in-

troduce a DC offset current into the inverting input of the

amplifier. To ensure minimal impact on frequency response

be sure to inject the DC offset current through large resistors.

Conversely if optimizing an inverting amplifier configuration

simply apply offset adjustment to the non-inverting input.

current by using larger values of gain (RF). The total capaci-

tance (CT) on the inverting terminal of the op amp includes

the photodiode capacitance (CPD) and the input capacitance

of the op amp (CIN). This total capacitance (CT) plays an im-

portant role in the stability of the circuit. The noise gain of this

circuit determines the stability and is defined by:

(1)

(2)

30033641

FIGURE 11. Current Sense Amplifier

TRANSIMPEDANCE AMPLIFIER

By definition, a photodiode produces either a current or volt-

age output from exposure to a light source. A Tran-

simpedance 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.

30033665

FIGURE 13. Bode Plot of Noise Gain Intersecting with Op

Amp Open Loop Gain

Figure 13 shows the bode plot of the noise gain intersecting

the op amp open loop gain. With larger values of gain, CT and

RF create a zero in the transfer function. At higher frequencies

the circuit can become unstable due to excess phase shift

around the loop.

A pole at fP in the noise gain function is created by placing a

feedback capacitor (CF) across RF. The noise gain slope is

flattened by choosing an appropriate value of CF for optimum

performance.

Theoretical expressions for calculating the optimum value of

CF and the expected â3 dB bandwidth are:

(3)

30033662

FIGURE 12. Photodiode Modeled with Capacitance

Elements

Figure 12 shows the LMH6611 modeled with photodiode and

the internal op amp capacitances. The LMH6611 allows cir-

cuit operation of a low intensity light due to its low input bias

(4)

Equation 4 indicates that the â3 dB bandwidth of the TIA is

inversely proportional to the feedback resistor. Therefore, if

the bandwidth is important then the best approach would be

to have a moderate transimpedance gain stage followed by a

broadband voltage gain stage.

Table 4 shows the measurement results of the LMH6611 with

different photodiodes having various capacitances (CPD) and

a feedback resistance (RF) of 1 kâ¦.

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