LMH6503 Datasheet, PDF (15/20 Pages) National Semiconductor (TI) – Wideband, Low Power, Linear Variable Gain Amplifier

Application Information (Continued)
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the minimum possible values that meet the input voltage and
maximum gain constraints. Scaling the resistor values will
decrease bandwidth and improve stability.
Figure 2 illustrates the resulting LMH6503 bandwidths as a
function of the maximum ( y axis) and minimum (related to x
axis) input voltages when V_OUT is held constant at 1V_PP.
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FIGURE 2. Bandwidth vs. V_INMAX and A_VMAX
ADJUSTING OFFSETS
Treating the offsets introduced by the input and output
stages of the LMH6503 is accomplished with a two step
process. The offset voltage of the output stage is treated by
first applying −1.1V on V_G, which effectively isolates the
input stage and multiplier core from the output stage. As
illustrated in Figure 3, the trim pot located at R14 on the
LMH6503 Evaluation Board (CLC730033) should then be
adjusted in order to null the offset voltage seen at the
LMH6503’s output (pin 10).
Once this is accomplished, the offset errors introduced by
the input stage and multiplier core can then be treated. The
second step requires the absence of an input signal and
matched source impedances on the two input pins in order to
cancel the bias current errors. This done, then +1.1V should
be applied to V_G and the trim pot located at R₁₀ adjusted in
order to null the offset voltage seen at the LMH6503’s output.
If a more limited gain range is anticipated, the above adjust-
ments should be made at these operating points. These
steps will minimize the output offset voltage. However, since
the offset term itself varies with the gain setting, the correc-
tion is not perfect and some residual output offset will re-
main.
GAIN ACCURACY
Defined as the ratio of measured gain (V/V), at a certain V_G,
to the best fit line drawn through the typical gain (V/V)
distribution for −1V < V_G < 1V (results expressed in dB)
(See Figure 4). The best fit gain (A_V) is given by:
A_V (V/V) = 4.87V_G + 4.61
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For: −1V ≤ V_G ≤ + 1V, R_F = 1kΩ, R_G = 174Ω
For a V_G range, the value specified in the tables represents
the worst case accuracy over the entire range. The "Typical"
value would be the worst case ratio between the "Typical
Gain" and the best fit line. The "Max" value would be the
worst case between the max/min gain limit and the best fit
line.
GAIN MATCHING
Defined as the limit on gain variation at a certain V_G (ex-
pressed in dB) (See Figure 4). Specified as "Max" only (no
"Typical"). For a V_G range, the value specified represents the
worst case matching over the entire range. The "Max" value
would be the worst case ratio between the max/min gain limit
and the typical gain.
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FIGURE 4. Gain Accuracy and Gain Matching
Parameters Defined
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FIGURE 3. Nulling the Output Offset Voltage
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