THS4561 Datasheet, PDF (29/35 Pages) Texas Instruments – Low-Power, High Supply Range, 70-MHz, Fully Differential Amplifier

www.ti.com
THS4561
SBOS874 – AUGUST 2017
10.2 Typical Application
One common application for the THS4561 is to take a single-ended, high V_PP voltage swing (from a high-voltage
precision amplifier such as the OPA192) and deliver that swing to precision SAR ADC as a single-ended to
differential conversion with output common-mode control and implement an active 2nd-order multiple feedback
(MFB) filter design. Designing for a 40-V_PP maximum input down to an 8-V_PP differential swing requires a gain of
0.2 V/V. Targeting a 100-kHz Butterworth response with the RC elements tilted towards low noise gives the
example design of Figure 12. The V_CM control is set to half of a 4.096-V reference, which is typical for
5-V differential SAR applications.
OPA192 Output
RF1
592
THS4561 Wideband,
Fully Differential Amplifier
VS+
VS±
+
5V
±
0V ⁺
±
VIN
VOCM
+
2.048 V
±
2.96 k
1.5 nF
2.96 k
RG1
1.57 k
910 pF
VS+
20
10
100 fF
VOCM
±
+
FDA
±
+
PD
VS± ^VS+
20
10
RG2
1.57 k
RF2
592
910 pF
100 pF
2.2 nF
8-VPP Differential
SAR ADC Input
100 pF
Copyright © 2017, Texas Instruments Incorporated
Figure 12. MFB Filter Driving an ADC Application:
Example 100-kHz Butterworth Response
10.2.1 Design Requirements
The requirements for this application are:
• Single-ended to differential conversion
• Attenuation by 0.2-V/V gain
• Active filter set to a Butterworth, 100-kHz response shape
• Output RC elements set by SAR input requirements (not part of the filter design)
• Filter element resistors and capacitors are set to limit added noise over the THS4561 and noise peaking
10.2.2 Detailed Design Procedure
The design proceeds using the techniques and tools suggested in the Design Methodology for MFB Filters in
ADC Interface Applications application note (SBOA114). The process includes:
• Scale the resistor values to not meaningfully contribute to the output noise produced by the THS4561 by itself
• Select the RC ratios to hit the filter targets when reducing the noise gain peaking within the filter design
• Set the output resistor to 10 Ω into a 2.2-nF differential capacitor
• Add 100-pF common-mode capacitors to the load capacitor to improve common noise filtering
• Inside the loop, add 20-Ω output resistors after the filter feedback capacitor to increase the isolation to the
load capacitor
• Include a place for a differential input capacitor (illustrated as 100 fF in Figure 12)
Copyright © 2017, Texas Instruments Incorporated
Product Folder Links: THS4561
Submit Documentation Feedback
29