LMH6502MTSLASHNOPB datasheet(16/29 Pages) TI

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RG =

RF x 1.72

AVMAX

- 3: WITH RF = 1K:

LMH6502

SNOSA65D – OCTOBER 2003 – REVISED MARCH 2013

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APPLICATION INFORMATION

THEORY OF OPERATION

A simplified schematic is shown in Figure 58. +VIN and −VIN are buffered with closed loop voltage followers

inducing a signal current in Rg proportional to (+VIN) - (−VIN), the differential input voltage. This current controls a

current source which supplies two well-matched transistor, Q1 and Q2.

The current flowing through Q2 is converted to the final output voltage using RF and the output amplifier, U1. By

changing the fraction of the signal current "I" which flows through Q2, the gain is changed. This is done by

changing the voltage applied differentially to the bases of Q1 and Q2. For example, with VG = 0V, Q1 conducts

heavily and Q2 is off. With none of "I" flowing through RF, the LMH6502's input to output gain is strongly

attenuated. With VG = +2V, Q1 is off and the entire signal current flows through Q2 to RF producing maximum

gain. With VG set to 1V, the bases of Q1 and Q2 are set to approximately the same voltage, Q1 and Q2 have the

same collector currents - equal to one half of the signal current "I", thus the gain is approximately one half the

maximum gain.

Figure 58. LMH6502 Block Diagram

CHOOSING RF & RG

Maximum input amplitude and maximum gain are the two key specifications that determine component values in

a LMH6502 application.

The output stage op amp is a current-feedback type amplifier optimized for RF = 1kΩ. RG can then be computed

as:

(1)

To determine whether the maximum input amplitude will overdrive the LMH6502, compute:

VDMAX = (RG + 3.0Ω) × 1.70mA

(2)

the maximum differential input voltage for linear operation. If the maximum input amplitude exceeds the above

VDMAX limit, then LMH6502 should either be moved to a location in the signal chain where input amplitudes are

reduced, or the LMH6502 gain AVMAX should be reduced or the values for RG and RF should be increased. The

overall system performance impact is different based on the choice made. If the input amplitude is reduced, re-

compute the impact on signal-to-noise ratio. If AVMAX is reduced, post LMH6502 amplifier gain, should be

increased, or another gain stage added to make up for reduced system gain. To increase RG and RF, compute

the lowest acceptable value for RG:

RG > 590 × VDMAX - 3Ω

(3)

Operating with RG larger than this value insures linear operation of the input buffers.

RF may be computed from selected RG and AVMAX: RF should be > = 1kΩ for overall best performance, however

RF < 1kΩ can be implemented if necessary using a loop gain reducing resistor to ground on the inverting

summing node of the output amplifier (see application note OA-13 (SNOA366) for details).

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