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Using Differential Amplifier Drive Circuits

FIL

0.1 F

10 F

||R

INP

INM

ADS6xxx

THS4509

VCM

500 W

200 W

5 W

500 W

0.1 F

–V

S0259-01

INPUT COMMON MODE

155 mAxFs

125 MSPS

(1)

REFERENCE

ADS6225

,, ADS6224

ADS6223, ADS6222

SLAS543A – MAY 2007 – REVISED SEPTEMBER 2007

Figure 84 shows a drive ciruit using a differential amplifier (TI's THS4509) to convert a single-ended input to

differential output that can be interfaced to the ADC input pins. In addition to the single-ended to differential

conversion, the amplifier also provides gain (10 dB in Figure 84). As shown in the figure, RFIL helps to isolate the

amplifier output from the switching inputs of the ADC. Together with CFIL, it also forms a low-pass filter that

bandlimits the noise (and signal) at the ADC input. As the amplifier outputs are ac-coupled, the common-mode of

the ADC input pins is set using two resistors connected to VCM. The amplifier outputs can also be dc-coupled.

Using the output common-mode control of the THS4509, the ADC input pins can be biased to 1.5 V.

Figure 84. Drive Circuit using THS4509

Refer to the EVM User Guide (SLAU196) for more information.

To ensure a low-noise common-mode reference, the VCM pin is filtered with a 0.1-

μF low-inductance capacitor

connected to ground. The VCM pin is designed to directly drive the ADC inputs. The input stage of the ADC

sinks a common-mode current in the order of 155

μA at 125 MSPS (per input pin). Equation 1 describes the

dependency of the common-mode current and the sampling frequency.

This equation helps to design the output capability and impedance of the CM driving circuit accordingly.

The ADS622X has built-in internal references REFP and REFM, requiring no external components. Design

schemes are used to linearize the converter load seen by the references; this and the on-chip integration of the

requisite reference capacitors eliminates the need for external decoupling. The full-scale input range of the

converter can be controlled in the external reference mode as explained below. The internal or external reference

modes can be selected by programming the register bit <REF> (Table 14).

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