THS4561 Datasheet, PDF (26/35 Pages) Texas Instruments – Low-Power, High Supply Range, 70-MHz, Fully Differential Amplifier

THS4561
SBOS874 – AUGUST 2017
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Application Information (continued)
Starting from a target feedback resistor value, the desired input matching impedance, and the target gain (A_V),
the required input R_T value is given by solving the quadratic of Equation 1.
R_T2
§
R_T
2R_F
2R_S
2
¨© ^2RF
A_V
R_S
2
A_V2
·
¸¹
R_SA_V (4 A_V )
2R_FR_S2 A _V
2R_F 2 A_V R_SA_V (4 A_V )
0
(1)
When this value is derived, the required input side gain resistor is given by Equation 2 and then the single value
for R_G2 on the non-signal input side is given by Equation 3:
R_G1
2 ^RF
A_V
R_S
1 ^RS
R_T
(2)
2 ^RF
R_G2
A_V
1 ^RS
R_T
(3)
Using these expressions to generate a swept gain table of values results in Table 1, where the best standard 1%
resistor values are shown to minimize input impedance and gain error to target.
GAIN (V/V)
0.1
1
2
5
10
Table 1. Swept Gain 50-Ω Input Match with R_F = 1.5-kΩ (±1 Standard Values)
R_F
1500
R_G1
15000
R_T
49.9
R_G2
15000
Z_IN
49.74
1500
1500
51.1
1500
49.82
1500
750
52.3
768
49.98
1500
287
54.9
316
49.6
1500
137
61.9
165
50.4
A_V
0.09973
0.994
1.978
5.014
10.08
Where an input impedance match is not required, simply set the input resistor to obtain the desired gain without
an additional resistor to ground (remove R_T in Figure 10). This scenario is common when coming from the output
of another single-ended op amp (such as the OPA192). This single-ended to differential stage shows a higher
input impedance than the physical R_G as given by the expression for Z_A (active input impedance) shown as
Equation 4.
§
¨¹
R_G1
·
¸
§
¨¹
R_F
·
¸
Z_A R_G1 ^©
R_{G2 ¹ ©} R_{G1 ¹}
2 ^RF
R_G2
(4)
Using Equation 4 for the gain of 1 V/V with all resistors equal to 1.5-kΩ shows an input impedance of 2 kΩ. The
increased input impedance comes from the common-mode input voltage at the amplifier pins moving in the same
direction as the input signal. The common-mode input voltage must move to create the current in the non-signal
input R_G resistor to produce the inverted output. The current flow into the signal-side input resistor is impeded
because the common-mode input voltage moves with the input signal, thus increasing the apparent input
impedance in the signal input path.
26
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