THS4541IRUNR.B datasheet(44/66 Pages) TI2

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8.4.2.2 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 8-5 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 that is well within range.

Rf1

499

Rg1

250

Rg2

250

THS4541 Wideband,

Fully-Differential Amplifier

Rf2

499

7.5 pF

30 pF

Low-Power

ADC3224

12-Bit, Dual,

125-MSPS

ADC

ADC Vcm

0.95-V Output

100-MHz,

Single-RC

Pole

Third-Order

50-MHz, 0.2-dB Ripple,

Chebychev, DC-Coupled

Low-Pass Filter

7.5 pF

920 nH

250

VG1

VG2

VS2

1.2 V

Quadrature Downconverter

DC Bias and

Output Impedance

Vcc

Vocm

Vcc

FDA

Figure 8-5. Example DC-Coupled, Differential I/O Design from a Quadrature Mixer to an ADC

THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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