LMH6503 datasheet(15/20 Pages) NSC

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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 1VPP.

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

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:

V (V/V) = 4.87VG + 4.61

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For: −1V

≤ V

≤ +1V, R

F =1k

Ω,R

G = 174

Ω

ForaV

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.

20073902

FIGURE 2. Bandwidth vs. V

INMAX and AVMAX

20073954

FIGURE 3. Nulling the Output Offset Voltage

20073955

FIGURE 4. Gain Accuracy and Gain Matching

Parameters Defined

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