A(V/V) = K x

RF

RG

x

1

1 - VG

1 + e

VC

LMH6502

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SNOSA65D – OCTOBER 2003 – REVISED MARCH 2013

ADJUSTING OFFSET

Offset can be broken into two parts; an input-referred term and an output-referred term. The input-referred offset

shows up as a variation in output voltage as VG is changed. This can be trimmed using the circuit in Figure 59 by

placing a low frequency square wave (VLOW = 0V, VHIGH = 2V into VG with VIN = 0V, the input referred VOS term

shows up as a small square wave riding a DC value. Adjust R10 to null the VOS square wave term to zero. After

adjusting the input-referred offset, adjust R14 (with VIN = 0, VG = 0) until VOUT is zero. Finally, for inverting

applications VIN may be applied to pin 6 and the offset adjustment to pin 3. These steps will minimize the output

offset voltage. However, since the offset term itself varies with the gain setting, the correction is not perfect and

some residual output offset will remain at in-between VG's. Also, this offset trim does not improve output offset

temperature coefficient.

Figure 59. Nulling the output offset voltage

GAIN ACCURACY

Defined as the actual gain compared against the theoretical gain at a certain VG (results expressed in dB).

Theoretical gain is given by:

(4)

Where K = 1.72 (nominal) & VC = 90mV @ room temperature.

For a VG range, the value specified in the tables represents the worst case accuracy over the entire range. The

"Typical" value would be the worst case difference between the "Typical Gain" and the "Theoretical gain". The

"Max" value would be the worst case difference between the max/min gain limit and the "Theoretical gain".

GAIN MATCHING

Defined as the limit on gain variation at a certain VG (expressed in dB). Specified as "Max" only (no "Typical").

For a VG range, the value specified represents the worst case matching over the entire range. The "Max" value

would be the worst case difference between the max/min gain limit and the typical gain.

NOISE

Figure 60 describes the LMH6502's output-referred spot noise density as a function of frequency with AVMAX =

10V/V. The plot includes all the noise contributing terms. However, with both inputs terminated in 50

Ω, the input

noise contribution is minimal. At AVMAX = 10V/V, the LMH6502 has a typical input-referred spot noise density (ein)

of 7.7nV/

flat-band. For applications extending well into the flat-band region, the input RMS voltage noise can

be determined from the following single-pole model:

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