THS4541 Datasheet, PDF (27/62 Pages) Texas Instruments – Fully Differential Amplifier

www.ti.com
THS4541
SLOS375 – AUGUST 2014
I/O Headroom Considerations (continued)
One method of deriving the voltage range for Vicm for any single-ended input to differential output design is to
determine the voltage swing on the nonsignal-input side of the FDA outputs and simply take its divider back to
the input pin to ground or the dc reference used on that side. An example analysis is shown in Figure 68, where
the circuit of Figure 61 is simplified to show just a Thevenized source impedance.
Vocm
Vcc
Rth
27.3
C1
100 nF
Rg1
191
Rf1
402
+ _Vcm
± 2.5 V
+ _Vs1
± 5V
+
Vs
±
THS4541
Vocm
C2
100 nF
Rg2
221
Vcc
±
+
FDA
±
+
PD
Vcc
Rf2
402
Figure 68. Input Swing Analysis Circuit from Figure 61 with Thevenized Source
For this ac-coupled input analysis, the nominal dc input Vicm is simply the output Vocm (2.5 V in this example
design). Then, considering the lower side of the feedback networks, any desired maximum output differential V_PP
generates a known ac V_PP at the junction of Rg2 and Rf2. For instance, if the design intends a maximum 4-V_PP
differential output, each FDA output pin is ±1 V around the Vocm (= 2.5 V), and then back to the Vicm, which
produces a ±1 V × 221 / (221 + 402) = ±0.355 V around the dc setting of Vocm. This simple approach to
assessing the input Vicm range for a single-ended to differential design can be applied to any design using an
FDA by reducing the input side circuits to a divider to either the signal source and ground or voltage reference on
the nonsignal input side.
Copyright © 2014, Texas Instruments Incorporated
Product Folder Links: THS4541
Submit Documentation Feedback
27