AD6657EBZ datasheet(17/32 Pages) AD

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AD6657

Rev. 0 | Page 17 of 32

Differential Input Configurations

Optimum performance is achieved when driving the AD6657

in a differential input configuration. For baseband applications,

the AD8138, ADA4937-2, and ADA4938-2 differential drivers

provide excellent performance and a flexible interface to the ADC.

The output common-mode voltage of the ADA4938-2 is easily

set with the VCMx pin of the AD6657 (see Figure 30), 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

ADA4938-2

Figure 30. Differential Input Configuration Using the ADA4938-2

For baseband applications where SNR is a key parameter,

differential transformer coupling is the recommended input

configuration. An example is shown in Figure 31. To bias the

analog input, the VCM voltage can be connected to the center

tap of the secondary winding of the transformer.

2V p-p

49.9Ω

0.1µF

ADC

VIN+

VIN–

VCM

Figure 31. Differential Transformer-Coupled Configuration

The signal characteristics must be considered when selecting

a transformer. Most RF transformers saturate at frequencies

below a few megahertz (MHz). 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 AD6657. For applications in

which SNR is a key parameter, differential double balun coupling

is the recommended input configuration (see Figure 32). In this

configuration, the input is ac-coupled and the CML 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 and may need to be reduced

or removed. Table 10 lists recommended values to set the RC

network. At higher input frequencies, good performance can be

achieved by using a ferrite bead in series with a resistor and

removing the capacitors. However, these values are dependent

on the input signal and should be used only as a starting guide.

Table 10. Example RC Network

Frequency

Range

(MHz)

R1 Series

(Each)

C1 Differential

R2 Series

(Each)

C2 Shunt

(Each)

0 to 100

33 Ω

5 pF

15 Ω

15 pF

100 to 200

10 Ω

5 pF

10 Ω

10 pF

100 to 300

10 Ω1

Remove

66 Ω

Remove

1 In this configuration, R1 is a ferrite bead with a value of 10 Ω @ 100 MHz.

An alternative to using a transformer-coupled input at frequencies

in the second Nyquist zone is to use the AD8352 differential driver

(see Figure 33). For more information, see the AD8352 data sheet.

ADC

0.1µF

2V p-p

VIN+

VIN–

VCM

0.1µF

33Ω

Figure 32. Differential Double Balun Input Configuration

AD8352

0Ω

0.1µF

8, 13

VCC

200Ω

ANALOG INPUT

ADC

VIN+

VIN–

VCM

Figure 33. Differential Input Configuration Using the AD8352

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