LM363 datasheet(11/22 Pages) NSC

11 / 22 page

Application Hints (Continued)

Because the LM363’s offset voltage is so low to begin with

offset nulling has a negligible effect on offset temperature

drift For example zeroing a 100 mV offset assuming external

resistor TC of 200 ppm C and worst-case internal resistor

TC results in an additional drift component of 008 mV C

For this reason drift specifications are guaranteed with or

without external offset nulling

GAIN ADJUSTMENT

Gain may be increased by adding an external voltage divid-

er between output force and sense and reference the pre-

ferred connection is shown in

Figure 4 Since both the

sense and reference pins look like 50 kX (g20 kX)toVb

impedances presented to both pins must be equal to avoid

offset error For example a 100X imbalance can create a

worst-case output offset of 50 mV creating an input-re-

ferred error of 5 mV at Ge10 or 50 mVatGe1000

Increasing gain this way increases output offset error An

LM363H-100 may have an output offset of 5 mV resulting in

input referred offset component of 50 mV Raising the gain

to 200 yields a 10 mV error at the output and changes input

referred error by an additional 50 mV

External resistors connected to the reference and sense

pins can only

increase the gain If ultra-low output imped-

ance is not critical the technique in

Figure 5 can be used to

trim the gain to nominal value Alternatively the VOS adjust-

ment terminals on the 16-pin package may be used to trim

the gain (

Figure 10b )

TLH5609 – 12

R1 and R2 should be as low as possible to avoid errors due to 50 kX

input impedance of reference and sense pins

Total resistance

(R2a2R1) should be above 4 kX however to prevent excessive load

on the LM363 output The exact formula for calculating gain (G) is

GeGO

1a2R1R2aR150kJ

GOepreset gain

The last term may be ignored in applications where gain accuracy is not

critical The table below gives suggested values for R1 and R2 along

with the calculated error due to ‘‘closest value’’ standard 1% resistors

Total gain error tolerance includes contributions from LM363 GO error

and resistor tolerance (g1%) and works out to approximately 25% in

every case

Pinout shown is for 16-pin package This same technique can also be

used with 8-pin versions

Gain Increase

1 5

2 5

6789

121k

178k

249k

294k

348k

392k

442k

249k

274k

205k

121k

Error (typ)

06%

02%

03%

06%

08%

05%

09%

04%

09%

07%

FIGURE 4 Increasing Gain

Pinout shown is for 8-pin versions

This same technique can also be used

with 16-pin version

TLH5609 – 13

FIGURE 5 Adjusting Gain Alternate Technique

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