THS4541 Datasheet, PDF (29/62 Pages) Texas Instruments – Fully Differential Amplifier

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THS4541
SLOS375 – AUGUST 2014
Output DC Error and Drift Calculations and the Effect of Resistor Imbalances (continued)
This conversion gain to differential output error is applied to two error terms: Vocm, assuming the input control
pin is driven and not floating, and the source Vcm voltage. The source common-mode voltage is assumed to be
0 V in this example. If not, apply this gain to the source common-mode value or range in the intended
application.
As a full example of using these terms to estimate the worst-case output 25°C error band, and then the worst-
case drift (by adding all the error terms together independently), use the gain of 2 V/V configuration of Figure 63
with Rf = 402 Ω, and assume ±1% tolerance on the resistors with the standard values used in Figure 69.
Vcc
+ Vcc
± ^5V
Vocm
+ _Vocm
± 2.5 V
50- Input-Matched
Gain of 2 V/V from Rt,
Single-Ended to Differential,
DC-Coupled, Single Supply
Rs
50
Rg1
191
+
Vs
±
Rt
Vocm
60.2
Rg2
221
Rf1
402
THS4541 Wideband,
Fully-Differential Amplifier
Vcc
±
+
FDA
±
+
PD
Vcc
R1
500
+
VM1
±
Rf2
402
Figure 69. DC-Coupled Gain of 2 with Rf = 402 Ω and a Single-Ended to Differential Matched Input 50-Ω
Impedance
The standard value snap on the signal-input side actually produces more G mismatch than the resistor
tolerances. For Figure 69, G2 = 402 / 221 = 1.819; and G1 = 402 / 218.3 = 1.837 nominally, with a ±2%
tolerance around this initial mismatch for G2 and G1, if 1% resistors are used.
Using the maximum 25°C error terms, and a nominal 2.5-V input to the Vocm control pin gives Table 3, gains to
the output differential error (Vod), and then the summed output error band at 25°C.
ERROR TERM
Input Vio
Input Ios
Input Ibcm, Rf mismatch
Vocm input, G mismatch
Table 3. Worst-Case Output Vod Error Band
25°C MAXIMUM VALUE
±0.45 mV
±0.5 µA
13 µA
2.5 V
GAIN TO Vod
2.85 V/V
402 Ω
±8.04 Ω
±0.0322
Total
OUTPUT ERROR
±1.2825 mV
±0.201 mV
±0.105 mV
±80.5 mV
±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 snap, and using a ±1% tolerance on the Rf and Rg elements of that ratio; the actual
G1 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 the term.
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