ADC3641 datasheet(24/77 Pages) TI

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8.3.1.2.3 Analog Input Termination and DC Bias

Depending on the input drive circuitry, a termination network and/or DC biasing needs to be provided.

8.3.1.2.3.1 AC-Coupling

The ADC364x requires external DC bias using the common mode output voltage (VCM) of the ADC together

with the termination network as shown in Figure 8-7. The termination is located within the glitch filter network.

When using a balun on the input, the termination impedance has to be adjusted to account for the turns ratio of

the transformer. When using an amplifier, the termination impedance can be adjusted to optimize the amplifier

performance.

100 pF

0.1 …F

180nH

VCM

Glitch Filter

1 uF

VCM

Termination

Figure 8-7. AC-Coupling: termination network provides DC bias (glitch filter example for DC - 30 MHz)

8.3.1.2.3.2 DC-Coupling

In DC coupled applications the DC bias needs to be provided from the fully differential amplifier (FDA) using

VCM output of the ADC as shown in Figure 8-8. The glitch filter in this case is located between the anti-alias filter

and the ADC. No termination may be needed if amplifier is located close to the ADC or if the termination is part

of the anti-alias filter.

100 pF

180nH

Glitch Filter

AAF (Anti

Alias Filter)

VCM

Figure 8-8. DC-Coupling: DC bias provided by FDA (glitch filter example for DC - 30 MHz)

ADC3643

SBAS886 – OCTOBER 2020

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