THS4551 Datasheet, PDF (34/70 Pages) Texas Instruments – Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

THS4551
SBOS778A – APRIL 2016 – REVISED AUGUST 2016
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The terms that create a differential error from a common-mode term and feedback ratio mismatch include the
desired V_OCM voltage, any source common-mode voltage, any drift on the reference bias to the V_OCM control pin,
any internal offset and drift in the V_OCM control path, and the input average bias current and drift.
Considering just the output common-mode control and the source common-mode voltage, the conversion to
output differential offsets is through Equation 8.
V_OD
V_OCM G1 G2 V_ICM G1 G2
1 ^{G1 G2}
2
(8)
Neglecting any G1 and G2 mismatch because of standard values constraint, the conversion gain for these two
terms can be recast in terms of the nominal R_F / R_G ≡ G and the tolerance T, as shown in Equation 9. When G
increases, this conversion gain approaches 4T.
V_OD
V_OCM
G
4T
(1
‡
G) ₍₁
T ²)
(9)
This conversion gain to differential output error is applied to two error terms: V_OCM and the input bias current and
drift. (The source common-mode voltage is assumed to be 0 V. If not, apply this gain to the source common-
mode voltage and any resulting shift in application.)
The output error is applied to V_OCM, assuming that the input control pin is driven and not floating. The input bias
current and drift are multiplied by the average R_F value then by the conversion gain to differential output error to
create an added output differential error.
As an example of using these terms to estimate the worst-case output 25°C error band and then the worst-case
drift (by adding all error terms together independently), use the gain of 1-V/V configuration with R_F = 1 kΩ and
assume a ±1% tolerance on the resistors with the standard values used in Figure 74.
VS+
+
5V
±
VS-
+
0V
±
VOCM
+
2.5 V
±
50-
Source
Impedance
50- Input Match,
Gain of 1 V/V from RT,
Single-Ended Source to
Differential Output
THS4551 Wideband,
Fully Differential Amplifier
RF1
1k
RS1
50
RG1
1k
RT1
52.3
VOCM
VS+
±
+
FDA
±
+
PD
RG2
1.02 k
VS- ^VS+
VOPP
RL
1k
RF2
1k
Copyright © 2016, Texas Instruments Incorporated
Figure 74. DC-Coupled Gain of 1 with R_F = 1 kΩ and Single-to-Differential Matched Input 50-Ω Impedance
The standard value constraint on the non-signal input side actually produces more gain mismatch than the
resistor tolerances. For Figure 74, G2 = 1000 / 1020 = 0.9804 and G1 = 1000 / 1025.6 = 0.9751 nominally, then
with a ±2% tolerance around the initial gain mismatch resulting from the standard values available 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 2, gains to
the output differential error (V_OD), and then the summed output error band at 25°C. The output error is clearly
dominated by the V_OCM voltage and the effect of the feedback dividers being slightly mismatched. This analysis
does not completely include resistor tolerances but the approach is the same with the wider error bands on the
gain terms. For the lowest output error, this analysis shows that an exact match on the feedback dividers with
precision resistors is preferred. However, doing so would require duplicating the exact network on the non-signal
input side and the signal input side. Where impedance termination is not required, the two R_G resistors are
simply single equal resistors and the gain mismatch is just from the tolerance of the resistors.
34
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