THS4541 Datasheet, PDF (43/62 Pages) Texas Instruments – Fully Differential Amplifier

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THS4541
SLOS375 – AUGUST 2014
9.4.2.2 DC-Coupled, Differential-Input to Differential-Output Design Issues
Operating the THS4541 with a dc-coupled differential input source is very simple and only requires that the input
pins stay in range of the dc common-mode operating voltage. One example is a dc-to-50-MHz quadrature down-
converter output. These outputs typically sit on a dc level with some internal source impedance to the external
loads. The example of Figure 76 shows a design using the THS4541 with a simple, passive RLC filter to the
inputs (the Rg elements act as the differential termination for the filter design). From the original source behind
the internal 250-Ω outputs, this circuit is a gain of 1 to the THS4541 output pins. The dc common-mode operating
voltage level shifts from the 1.2-V internal, to the mixer, to an output at the ADC Vcm voltage of 0.95 V. In this
case, a simple average of the two dc voltages in the gain of 1 stage gives a 1.08-V input pin common-mode
result well within range.
Quadrature Downconverter
DC Bias and
Output Impedance
VG1
R2
250
VG2
R3
250
+ _VS2
± ^{1.2 V}
Third-Order
50-MHz, 0.2-dB Ripple,
Chebychev, DC-Coupled
Low-Pass Filter
L1
920 nH
Rg1
250
C1
7.5 pF
L2
920 nH
C2
7.5 pF
Rg2
250
THS4541 Wideband,
Fully-Differential Amplifier
Rf1
499
Vcc
Vocm
±
+
FDA
±
+
PD
Vcc
100-MHz,
R1
22
Single-RC
Pole
R4
22
C3
30 pF
Low-Power
ADC3224
12-Bit, Dual,
125-MSPS
ADC
Rf2
499
ADC Vcm
0.95-V Output
Figure 76. Example DC-Coupled, Differential I/O Design from a Quadrature Mixer to an ADC
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