THS4521 Datasheet, PDF (38/65 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

THS4521, THS4522, THS4524
SBOS458H – DECEMBER 2008 – REVISED JUNE 2015
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8.4.1.4 Input Impedance for the Single-Ended to Differential FDA Configuration
The designs so far have included a source impedance, R_S, that must be matched by R_T and R_G1. The total
impedance at the junction of R_T and R_G1 for the circuit of Figure 75 is the parallel combination of R_T to ground,
and the ZA (active impedance) presented by R_G1. The expression for ZA, assuming R_G2 is set to obtain the
differential divider balance, is given by Equation 5:
ZA
=
R _G1
æ
ççè1 +
R _G1
R _G2
ö
÷÷ø
æ
ççè1
+
2 + ^RF
R_F
R _G1
ö
÷÷ø
R _G2
(5)
For designs that do not need impedance matching, for instance where the input is driven from the low-impedance
output of another amplifier, R_G1 = R_G2 is the single-to-differential design used without an R_T to ground. Setting
R_G1 = R_G2 = R_G in Equation 5 produces Equation 6, which is the input impedance of a simple-input FDA driven
from a low-impedance, single-ended source.
ZA
=
2R _G
æ
ççè1
+
R_F
R_G
2 + ^RF
ö
÷÷ø
R_G
(6)
In this case, setting a target gain as R_F / R_G ≡ α, and then setting the desired input impedance allows the R_G
element to be resolved first. Then the R_F is set to get the target gain. For example, targeting an input impedance
of 200 Ω with a gain of 4 V/V, Equation 7 calculates the R_G value. Multiplying this required R_G value by a gain
of 4 gives the R_F value and the design of Figure 76.
R_G
=
ZA
2+a
2(1 + a)
(7)
THS452x Wideband,
Fully-Differential Amplifier
200-Ω Input Impedance
Gain of 4 V/V Design
Rg1
120 Ω
+
Vs
–
Vocm
Rf1
480 Ω
Vcc
–
+
FDA _–
+
PD
Rg2
Vcc
120 Ω
R1
500 Ω
Output
Measurement
Point
Rf2
480 Ω
Figure 76. 200-Ω Input Impedance, Single-Ended to Differential DC-Coupled Design With Gain of 4 V/V
After being designed, this circuit can also be ac-coupled by adding blocking caps in series with the two 120-Ω R_G
resistors. This active input impedance has the advantage of increasing the apparent load to the prior stage using
lower resistors values, leading to lower output noise for a given gain target.
38
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