Datasheet

LMH6502
SNOSA65D –OCTOBER 2003–REVISED MARCH 2013 www.ti.com
OPERATING AT LOWER SUPPLY VOLTAGES
The LMH6502 is rated for operation down to 5V supplies (V
+
-V
−
). There are some specifications shown for
operation at ±2.5V within the data sheet (i.e. Frequency Response, CMRR, PSRR, Gain vs. V
G
, etc.). Compared
to ±5V operation, at lower supplies:
a) V
G
range shifts lower.
Here are the approximate expressions for various V
G
voltages as a function of V
+
:
Table 1. V
G
Definition Based on V
+
V
G
Definition Expression (V)
V
G_MIN
Gain Cut-off 0.2 × V
+
−1
V
G_MID
A
VMAX
/2 0.2 × V
+
V
G_MAX
A
VMAX
0.2 × V
+
+1
b) V
G_LIMIT
(maximum permissible voltage on V
G
) is reduced. This is due to limitations within the device
arising from transistor headroom. Beyond this limit, device performance will be affected (non-destructive).
This could reveal itself as premature high frequency response roll-off. With ±2.5V supplies, V
G_LIMIT
is
below 1.1V whereas V
G
= 1.5V is needed to get maximum gain. This means that operating under these
conditions has reduced the maximum permissible voltage on V
G
to a level below what is needed to get
Max gain. If supply voltages are asymmetrical with V
+
being lower, further "pinching" of V
G
range could
result; for example, with V
+
= 2V, and V
−
= −3V, V
G_LIMIT
= 0.40V which results in maximum gain being
2.5dB less than what would be expected when V
S
is higher.
c) "Max_gain" reduces. There is an intrinsic reduction in max gain when the total supply voltage is reduced
(see Typical Performance Characteristics plots for Gain vs. V
G
(V
S
= ±2.5V). In addition, there is the more
drastic mechanism described in "b" above. Beyond V
G_LIMIT
, high frequency response is also effected.
Application Circuits
AGC LOOP
Figure 63 shows a typical AGC circuit. The LMH6502 is followed up with a LMH6714 for higher overall gain. The
output of the LMH6714 is rectified and fed to an inverting integrator using a LMH6657 (wideband voltage
feedback op amp). When the output voltage, V
OUT
, is too large the integrator output voltage ramps down
reducing the net gain of the LMH6502 and V
OUT
. If the output voltage is too small, the integrator ramps up
increasing the net gain and the output voltage. Actual output level is set with R
1
. To prevent shifts in DC output
voltage with DC changes in input signal level, trim pot R
2
is provided. AGC circuits are always limited in the
range of input signals over which constant output level can be maintained. In this circuit, we would expect that
reasonable AGC action could be maintained for at least 40dB. In practice, rectifier dynamic range limits reduce
this slightly.
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