THS4541_15 Datasheet, PDF (30/61 Pages) Texas Instruments – THS4541 Negative Rail Input, Rail-to-Rail Output, Precision, 850-MHz Fully Differential Amplifier

THS4541
SLOS375A – AUGUST 2014 – REVISED SEPTEMBER 2014
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Normally, the expected drift in the output Vod is of more interest than an initial error band. Table 4 shows these
terms and the summed results, adding all the terms independently to obtain a worst-case drift.
ERROR TERM
Input Vio
Input Ios
Input Ibcm, Rf mismatch
Vocm input, G mismatch
Table 4. Worst-Case Output Vod Drift Band
DRIFT MAXIMUM VALUE
±2.4 µV/°C
±1.3 nA/°C
15 nA/°C
±12 µV/°C
GAIN TO Vod
2.85 V/V
402 Ω
±8.04 Ω
±0.0322
Total
OUTPUT ERROR
±6.84 µV/°C
±0.522 µV/°C
±0.121 µV/°C
±0.386 µV/°C
±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.
A less conservative estimate of output drift considers the terms to be uncorrelated and RMS half of each terms
worst-case span shown in Table 4. 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.
8.5 Noise Analysis
The first step in the output noise analysis is to reduce the application circuit to its simplest form with equal
feedback and gain setting elements to ground, as shown in Figure 70, with the FDA and resistor noise terms to
be considered.
e_nRg2
Rg
e_nRf2
Rf
I_n+2
+
I_n±²
±
e_no2
e_nRg2
Rg
e_ni2
e_nRf2
Rf
Figure 70. FDA Noise-Analysis Circuit
The noise powers are shown for each term. When the Rf and Rg terms are matched on each side, the total
differential output noise is the RSS of these separate terms. Using NG ≡ 1 + Rf / Rg, the total output noise is
given by Equation 6. Each resistor noise term is a 4kTR power.
e_no e_niNG²  2i_nRf ²  24kTRfNG
(6)
30
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