AD9237 datasheet(17/28 Pages) AD

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AD9237

Rev. 0 | Page 17 of 28

The output common mode of the reference buffer is set to mid-

supply, and the REFT and REFB voltages and input span are

defined as:

REFT = ½(AVDD + VREF)

REFB = ½(AVDD − VREF)

()

Factor

Span

VREF

Factor

Span

REFB

REFT

Span

−

The previous equations show that the REFT and REFB voltages

are symmetrical about the midsupply voltage, and the input

span is proportional to the value of the VREF voltage, see Table 7

for more details.

The internal voltage reference can be pin strapped to fixed

values of 0.5 V or 1.0 V, or adjusted within this range as

discussed in the Internal Reference Connection section.

Maximum SNR performance is achieved with the AD9237

set to an input span of 2 V p-p or greater. The relative SNR

degradation is 3 dB when changing from 2 V p-p mode to

1 V p-p mode.

The SHA must be driven from a source that keeps the signal

peaks within the allowable range for the selected reference

voltage. The minimum and maximum common-mode input

levels are defined as:

VCMMIN = VREF/2

VCMMAX = (AVDD + VREF)/2

The minimum common-mode input level allows the AD9237 to

accommodate ground-referenced inputs.

Although optimum performance is achieved with a differential

input, a single-ended source can be driven into VIN+ or VIN–.

In this configuration, one input accepts the signal while the

opposite input should be set to midscale by connecting it to an

appropriate reference. For example, a 2 V p-p signal can be

applied to VIN+ while a 1 V reference is applied to VIN–. The

AD9237 then accepts an input signal varying between 2 V and

0 V. In the single-ended configuration, distortion performance

may degrade significantly as compared to the differential case.

However, the effect is less noticeable at lower input frequencies and

in the lower speed grade models (AD9237-40 and AD9237-20).

Differential Input Configurations

As previously detailed, optimum performance is achieved while

driving the AD9237 in a differential input configuration. For

baseband applications, the AD8351 differential driver provides

excellent performance and a flexible interface to the ADC. The

output common-mode voltage of the AD8351 is easily set to

AVDD/2, and the driver can be configured in a Sallen-Key filter

topology to provide band limiting of the input signal. Figure 36

details a typical configuration using the AD8351.

AD8351

AD9237

VIN+

AVDD

AGND

VIN–

–

0.1

μF

0.1

μF

15pF

0.1

μF

0.1

μF

1.2k

49.9

2V p-p

Figure 36. Differential Input Configuration Using the AD8351

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

performance of most amplifiers is not adequate to achieve the

true performance of the AD9237. This is especially true in IF

undersampling applications where frequencies in the 70 MHz

to 100 MHz range are being sampled. For these applications,

differential transformer coupling is the recommended input

configuration, as shown in Figure 37.

AD9237

VIN+

AVDD

AGND

VIN–

0.1

μF

15pF

2V p-p

49.9

Figure 37. Differential Transformer-Coupled Configuration

The signal characteristics must be considered when selecting a

transformer. Most RF transformers saturate at frequencies

below a few MHz, and excessive signal power can cause core

saturation, which leads to distortion.

Single-Ended Input Configuration

A single-ended input can provide adequate performance in

cost-sensitive applications. In this configuration, there is

degradation in SFDR and distortion performance due to the

large input common-mode swing. However, if the source

impedances on each input are matched, there should be little

effect on SNR performance. Figure 38 details a typical single-

ended input configuration.

AD9237

VIN+

AVDD

AGND

VIN–

0.1

μF

15pF

0.1

μF

49.9

2V p-p

Figure 38. Single-Ended Input Configuration

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