THS4561 Datasheet, PDF (23/35 Pages) Texas Instruments – Low-Power, High Supply Range, 70-MHz, Fully Differential Amplifier

www.ti.com
THS4561
SBOS874 – AUGUST 2017
Device Functional Modes (continued)
9.4.3.1 AC-Coupled, Differential-Input to Differential-Output Design Issues
The most common way to use the THS4561 with an ac-coupled differential source is to simply couple the input
into the R_G resistors through the blocking capacitors. Figure 6 shows a typical blocking capacitor approach to a
differential input. An optional input differential termination resistor (R_M) is included in this design. The R_M element
allows the input R_G resistors to be scaled up and still delivers lower differential input impedance to the source. In
this example, the R_G elements sum to show a 1-kΩ differential impedance and the R_M element combines in
parallel to provide a net 500-Ω ac differential impedance to the source. Again, the design ideally proceeds by
selecting the R_F element values, then the R_G to set the differential gain, and then an R_M element (if needed) to
achieve a target input impedance. Alternatively, the R_M element can be eliminated, with the 2 × R_G elements set
to the desired input impedance and R_F set to obtain the differential gain (equal to R_F / R_G).
THS4561 Wideband,
Fully Differential Amplifier _RF1
1.02 k
Differential I/O
with AC-Coupled
VS+
VS±
Input
+
+
3.3 V
0V
±
±
VIN
10 nF
RF1
1.02 k
10 nF
RG1
499
1 µF
VS+
VOCM
±
+
FDA
±
+
PD
VS- ^VS+
VOUT
RL
1k
RG2
499
RF2
1.02 k
Copyright © 2017, Texas Instruments Incorporated
Figure 6. Example AC-Coupled Differential Input Design
The dc biasing for an ac-coupled differential input design is very simple. The output V_OCM is set by the input
control voltage and, because there is no dc current path for the output common-mode voltage (as long as R_M is
only differential and not split and connected to ground for instance), the dc bias also sets the common-mode
operating points for the input pins. For a purely differential input, the voltages on the input pins remain fixed at
the output V_OCM setting and do not move with the input signal (unlike the single-ended input configurations where
the input pin common-mode voltages do move with the input signal). The TINA-TI™ simulation file is available for
Figure 6.
Copyright © 2017, Texas Instruments Incorporated
Product Folder Links: THS4561
Submit Documentation Feedback
23