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AD9634

Data Sheet

Rev. B | Page 20 of 30

Differential Input Configurations

Optimum performance can be achieved when driving the

AD9634 in a differential input configuration. For baseband

applications, the AD8138, ADA4937-1, and ADA4930-1

differential drivers provide excellent performance and a flexible

interface to the ADC.

The output common-mode voltage of the ADA4930-1 is easily

set with the VCM pin of the AD9634 (see Figure 47), and the

driver can be configured in a Sallen-Key filter topology to

provide band limiting of the input signal.

VIN

76.8Ω

120Ω

0.1µF

200Ω

90Ω

AVDD

33Ω

15Ω

5pF

15pF

ADC

VIN–

VIN+

VCM

ADA4930-1

0.1µF

Figure 47. Differential Input Configuration Using the ADA4930-1

For baseband applications where SNR is a key parameter,

differential transformer coupling is the recommended input

configuration. An example is shown in Figure 48. To bias the

analog input, connect the VCM voltage to the center tap of the

secondary winding of the transformer.

2V p-p

49.9Ω

0.1µF

ADC

VIN+

VIN–

VCM

0.1µF

Figure 48. Differential Transformer-Coupled Configuration

The signal characteristics must be considered when selecting

a transformer. Most RF transformers saturate at frequencies

below a few megahertz. Excessive signal power can also cause

core saturation, which leads to distortion.

At input frequencies in the second Nyquist zone and above, the

noise performance of most amplifiers is not adequate to achieve

the true SNR performance of the AD9634. For applications where

SNR is a key parameter, differential double balun coupling is

the recommended input configuration (see Figure 49). In this

configuration, the input is ac-coupled and the VCM voltage is

provided to each input through a 33 Ω resistor. These resistors

compensate for losses in the input baluns to provide a 50 Ω

impedance to the driver.

In the double balun and transformer configurations, the value

of the input capacitors and resistors is dependent on the input

frequency and source impedance. Based on these parameters

the value of the input resistors and capacitors may need to be

adjusted, or some components may need to be removed. Table 9

displays recommended values to set the RC network for different

input frequency ranges. However, these values are dependent on

the input signal and bandwidth and should be used only as a

starting guide. Note that the values given in Table 9 are for the

R1, R2, R3, C1, and C2 components shown in Figure 49.

Table 9. Example RC Network

Frequency Range (MHz)

R1 Series (Ω)

C1 Differential (pF)

R2 Series (Ω)

C2 Shunt (pF)

R3 Shunt (Ω)

0 to 100

8.2

49.9

100 to 300

3.9

8.2

49.9

ADC

0.1µF

2V p-p

VIN+

VIN–

VCM

0.1µF

33Ω

0.1µF

Figure 49. Differential Double Balun Input Configuration

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