ADS6445 datasheet(50/75 Pages) STMICROELECTRONICS

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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)

ADS6445, ADS6444

ADS6443, ADS6442

SLAS531 – MAY 2007

APPLICATION INFORMATION (continued)

Figure 87 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 87). 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

voltage of the ADC input spins 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. In this case, use +4 V and -1 V supplies for the THS4509 to ensure that it's

output common-mode voltage (1.5 V) is at mid-supply.

Figure 87. 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.

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