THS4541_15 Datasheet, PDF (44/61 Pages) Texas Instruments – THS4541 Negative Rail Input, Rail-to-Rail Output, Precision, 850-MHz Fully Differential Amplifier

THS4541
SLOS375A – AUGUST 2014 – REVISED SEPTEMBER 2014
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10 Application And Implementation
10.1 Application Information
The THS4541 offers an effective solution over a broad range of applications. Two examples are developed here.
First, an attenuator stage that directly receives a higher input signal voltage and translates it to a lower
differential swing on a fixed common-mode is shown. This design requires some attention to frequency-response
flatness issues, and one approach to managing these issues is shown. The second example is a gain of 2 V/V,
matched input of 50 Ω to an output set to 0.95 V common-mode followed by a third-order Bessel filter with
approximately 20 MHz of bandwidth feeding into the ADC34J22, a low-power, 12-bit, quad 50-MSPS JESD 204B
ADC.
10.2 Typical Applications
10.2.1 Designing Attenuators
Vcc
+ _Vcc
± 5V
Gain of 0.1 V/V from Rg1,
Single-Ended to Differential,
DC-Coupled, Single Supply
THS4541 Wideband,
Fully-Differential Amplifier
Rf1
402
Vcm
+ _Vcm
± 2.5 V
Rg1
4.02 k
+
VG1
±
Vocm
Vcc
±
+
FDA
±
+
PD
Rg2
Vcc
4.02 k
R1
500
+
VM1
±
Rf2
402
Figure 77. Divide-by-10 Attenuator Application for the THS4541
10.2.1.1 Design Requirements
In this design, the aim is to:
1. Present a 4-kΩ input impedance to a ±40-V input signal (maximum ±10 mA from the prior stage).
2. Attenuate that swing by a factor 1/10 (–20 dB) to a differential output swing.
3. Place that swing on a 2.5-V common-mode voltage at the THS4541 outputs.
4. Operate on a single +5-V supply and ground.
5. Tune the frequency response to a flat Butterworth response with external capacitors.
10.2.1.2 Detailed Design Procedure
Operating the THS4541 at a low dc noise gain, or with higher feedback resistors, can cause a lower phase
margin to exist, giving the response peaking shown in Figure 1 for the gain of 0.1 (a 1/10 attenuator) condition.
Although it is often useful operating the THS4541 as an attenuator (taking a large input range to a purely
differential signal around a controlled-output, common-mode voltage), the response peaking illustrated in
Figure 1 is usually undesirable. Several methods can be used to reduce or eliminate this peaking; usually, at the
cost of higher output noise. Using dc techniques always increases the output noise broadband, while using an ac
noise-gain-shaping technique peaks the noise, but only at higher frequencies that can then be filtered off with the
typical passive filters often used after this stage. Figure 77 shows a simplified schematic for the gain of 0.1 V/V
test from Figure 61.
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