LMH6502MASLASHNOPB datasheet(20/29 Pages) TI

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LMH6502

SNOSA65D – OCTOBER 2003 – REVISED MARCH 2013

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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. VG, etc.). Compared

to ±5V operation, at lower supplies:

VG range shifts lower.

Here are the approximate expressions for various VG voltages as a function of V

Table 1. VG Definition Based on V

Definition

Expression (V)

VG_MIN

Gain Cut-off

0.2 × V+

−1

VG_MID

AVMAX/2

0.2 × V+

VG_MAX

AVMAX

0.2 × V+ +1

VG_LIMIT (maximum permissible voltage on VG) 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, VG_LIMIT is

below 1.1V whereas VG = 1.5V is needed to get maximum gain. This means that operating under these

conditions has reduced the maximum permissible voltage on VG to a level below what is needed to get

Max gain. If supply voltages are asymmetrical with V+ being lower, further "pinching" of VG range could

result; for example, with V+ = 2V, and V− =

−3V, VG_LIMIT = 0.40V which results in maximum gain being

2.5dB less than what would be expected when VS is higher.

"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. VG (VS = ±2.5V). In addition, there is the more

drastic mechanism described in "b" above. Beyond VG_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, VOUT, is too large the integrator output voltage ramps down

reducing the net gain of the LMH6502 and VOUT. 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 R1. To prevent shifts in DC output

voltage with DC changes in input signal level, trim pot R2 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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