THS4551 Datasheet, PDF (35/70 Pages) Texas Instruments – Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

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THS4551
SBOS778A – APRIL 2016 – REVISED AUGUST 2016
ERROR TERM
Input V_IO
Input I_OS
Input I_BCM
Input I_BCM
V_OCM input
Table 2. Worst-Case Output V_OD Error Band
25°C MAX VALUE
GAIN TO V_OD
OUTPUT ERROR
(mV)
GAIN COMMENT
±0.175 mV
1.9777
±0.346
Average noise gain
±50 nA
1000
±0.05
Feedback resistor
1.5 µA
20 Ω
±0.03
Feedback resistor mismatch
1.5 µA
1 kΩ × 0.00268
±0.004
Converted to differential by gain mismatch
2.5 V
0.00268
Total
±12.5
±12.93
V_OCM to differential by gain mismatch
The 0.00268 conversion gain for the gain ratio mismatch is the worst-case ratio starting from the initially higher
G1 value resulting from the standard value constraint and using a ±1% tolerance on the R_F and R_G elements of
the ratio. Mismatch is not symmetric but is shown that way in the analysis.
Normally, the expected drift in the output V_OD is of more interest than an initial error band. Table 3 shows these
terms and the summed results by adding all terms independently to obtain a worst-case drift.
ERROR TERM
Input V_IO
Input I_OS
Input I_BCM
Input I_BCM
V_OCM input
Table 3. Worst-Case Output V_OD Drift Band
MAX VALUE
GAIN TO V_OD
OUTPUT ERROR
(µV/°C)
GAIN COMMENT
±2.0 µV/°C
1.9777
±3.95
Average noise gain
±280 pA/°C
1000
±0.28
Feedback resistor
5.5 nA/°C
20 Ω
±0.11
Feedback resistor mismatch
5.5 nA/°C
1 kΩ × 0.00268
±0.015
Converted to differential by gain mismatch
±10 µV/°C
0.00268
Total
±0.027
±4.38
V_OCM to differential by gain mismatch
In Table 3, the input offset voltage drift dominates the output drift. For the last term, the drift for the V_OCM path is
just for the internal offset drift of the common-mode path with a driven input. Any added external drift on the
source of the V_OCM input must also be considered. This type of calculation can be repeated for the exact
application circuit considering each of these terms in the context of a specific design.
The absolute accuracy and drift for the THS4551 are exceptionally good. Mismatched resistor feedback ratios
combined with a high drift in the V_OCM control input can actually dominate the output V_OD drift. Where the output
differential precision is more important than the input matching accuracy, consider matching the networks on the
two sides of the input to obtain improved nominal G1 to G2 match. The gains for the input bias current error
terms are relatively low when using the 1-kΩ feedback values. Higher R_F values provide the input-current-related
drift terms more gain.
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