Datasheet

R
G
=
R
F
x 1.72
A
VMAX
- 3: WITH R
F
= 1K:
LMH6502
SNOSA65D –OCTOBER 2003–REVISED MARCH 2013 www.ti.com
APPLICATION INFORMATION
THEORY OF OPERATION
A simplified schematic is shown in Figure 58. +V
IN
and −V
IN
are buffered with closed loop voltage followers
inducing a signal current in Rg proportional to (+V
IN
) - (−V
IN
), 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 R
F
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 V
G
= 0V, Q1 conducts
heavily and Q2 is off. With none of "I" flowing through R
F
, the LMH6502's input to output gain is strongly
attenuated. With V
G
= +2V, Q1 is off and the entire signal current flows through Q2 to R
F
producing maximum
gain. With V
G
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 R
F
& R
G
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 R
F
= 1kΩ. R
G
can then be computed
as:
(1)
To determine whether the maximum input amplitude will overdrive the LMH6502, compute:
V
DMAX
= (R
G
+ 3.0Ω) × 1.70mA (2)
the maximum differential input voltage for linear operation. If the maximum input amplitude exceeds the above
V
DMAX
limit, then LMH6502 should either be moved to a location in the signal chain where input amplitudes are
reduced, or the LMH6502 gain A
VMAX
should be reduced or the values for R
G
and R
F
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 A
VMAX
is reduced, post LMH6502 amplifier gain, should be
increased, or another gain stage added to make up for reduced system gain. To increase R
G
and R
F
, compute
the lowest acceptable value for R
G
:
R
G
> 590 × V
DMAX
- 3Ω (3)
Operating with R
G
larger than this value insures linear operation of the input buffers.
R
F
may be computed from selected R
G
and A
VMAX
: R
F
should be > = 1kΩ for overall best performance, however
R
F
< 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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