AD9231 datasheet(20/36 Pages) AD

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AD9231

Rev. 0 | Page 20 of 36

Input Common Mode

The analog inputs of the AD9231 are not internally dc-biased.

Therefore, in ac-coupled applications, the user must provide a

dc bias externally. Setting the device so that VCM = AVDD/2 is

recommended for optimum performance, but the device can

function over a wider range with reasonable performance, as

shown in Figure 39 and Figure 40.

An on-board, common-mode voltage reference is included in

the design and is available from the VCM pin. The VCM pin

must be decoupled to ground by a 0.1 μF capacitor, as described

in the Applications Information section.

100

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

INPUT COMMON-MODE VOLTAGE (V)

SFDR (dBc)

SNR (dBFS)

Figure 39. SNR/SFDR vs. Input Common-Mode Voltage,

fIN = 32.1 MHz, fS = 80 MSPS

100

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

INPUT COMMON-MODE VOLTAGE (V)

SFDR (dBc)

SNR (dBFS)

Figure 40. SNR/SFDR vs. Input Common-Mode Voltage,

fIN = 10.3 MHz, fS = 20 MSPS

Differential Input Configurations

Optimum performance is achieved while driving the AD9231 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 VCM pin of the AD9231 (see Figure 41), and the

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

provide band limiting of the input signal.

AVDD

VIN

76.8

Ω

120

Ω

0.1µF

Ω

10pF

200

Ω

200

Ω

ADA4938

ADC

VIN–x

VIN+x

VCM

Figure 41. Differential Input Configuration Using the ADA4938-2

For baseband applications below ~10 MHz where SNR is a key

parameter, differential transformer-coupling is the recommended

input configuration. An example is shown in Figure 42. 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

49.9

Ω

0.1µF

ADC

VCM

VIN+x

VIN–x

Figure 42. 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 AD9231. For applications above

~10 MHz where SNR is a key parameter, differential double balun

coupling is the recommended input configuration (see Figure 44).

An alternative to using a transformer-coupled input at frequencies

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

An example is shown in Figure 45. See the AD8352 data sheet

for more information.

In any configuration, the value of Shunt Capacitor C is dependent

on the input frequency and source impedance and may need to

be reduced or removed. Table 9 displays the suggested values to set

the RC network. However, these values are dependent on the

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

Table 9. Example RC Network

Frequency Range (MHz)

R Series

(Ω Each)

C Differential (pF)

0 to 70

70 to 200

125

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