THS4541IRUNR.B datasheet(29/66 Pages) TI2

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Table 7-3. Worst-Case Output Vod Error Band

ERROR TERM

25°C MAXIMUM VALUE

GAIN TO Vod

OUTPUT ERROR

Input Vio

±0.45 mV

2.85 V/V

±1.2825 mV

Input Ios

±0.5 µA

402 Ω

±0.201 mV

Input Ibcm, Rf mismatch

13 µA

±8.04 Ω

±0.105 mV

Vocm input, G mismatch

2.5 V

±0.0322

±80.5 mV

Total

±82.09 mV

The 0.03222 conversion gain for the G ratio mismatch is the worst case, starting from the initially higher G1

value because of standard value approximation, and using a ±1% tolerance on the Rf and Rg elements of

that ratio. The actual Vocm conversion gain range is not symmetric, but is shown that way here. The initial

25°C worst-case error band is dominated by the Vocm conversion to Vod through the feedback resistor ratio

mismatch. Improve this G match and tolerances to reduce this term.

Normally, the expected drift in the output Vod is of more interest than an initial error band. Table 7-4 shows these

terms and the summed results, adding all the terms independently to obtain a worst-case drift.

Table 7-4. Worst-Case Output Vod Drift Band

ERROR TERM

DRIFT MAXIMUM VALUE

GAIN TO Vod

OUTPUT ERROR

Input Vio

±2.4 µV/°C

2.85 V/V

±6.84 µV/°C

Input Ios

±1.3 nA/°C

402 Ω

±0.522 µV/°C

Input Ibcm, Rf mismatch

15 nA/°C

±8.04 Ω

±0.121 µV/°C

Vocm input, G mismatch

±12 µV/°C

±0.0322

±0.386 µV/°C

Total

±7.86 µV/°C

In this calculation, the input offset voltage drift dominates the output differential offset drift. For the last term, the

drift for the Vocm path is just for the internal offset drift of the common-mode path. Make sure to also consider

the added external drift on the source of the Vocm input.

The absolute accuracy and drift for the THS4541 are exceptionally good. Mismatched resistor feedback ratios

combined with a high drift in the Vocm control input can actually dominate the output Vod drift. Where the output

differential precision is more important than the input matching accuracy, consider matching the networks on the

two input sides to achieve improved nominal G1 to G2 match. The gains for the input bias current error terms are

relatively low in this example design using 402-Ω feedback values. Higher Rf values give these terms more gain.

Table 7-4 shows a less conservative estimate of output drift considers the terms to be uncorrelated and RMS half

of each terms worst-case span. Performing this calculation for this example estimates a less conservative output

offset drift of ±3.42 µV/°C; essentially, half the worst-case span of the input offset drift term. Follow these steps to

estimate the output differential offset and drift for any external configuration.

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THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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