AD6652BBC datasheet(25/76 Pages) AD

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AD6652

Rev. 0 | Page 25 of 76

e source impedances driving

For best dynamic performance, th

the differential analog inputs should be matched such that

common-mode settling errors are symmetrical. These errors are

reduced by the common-mode rejection of the ADC.

5pF

S = SAMPLE

H = HOLD

VINA+

VINA–

Figure 38. Switched-Capacitor SHA Input for One ADC Channel

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

peak

voltage.

put

52 to

ential

put, a single-ended source can be driven into VIN+ or VIN−.

n 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

AD6652 then accepts a signal varying between 2 V and 0 V. In

the single-ended configuration, distortion performance might

degrade significantly, compared to the differential case.

However, the effect is less noticeable at lower analog input

frequencies.

Differential Input Configurations

Optimum performance is achieved while driving the AD6652

inputs in a differential input configuration. For baseband

applications to Nyquist, the AD8138 Differential Driver

provides excellent performance and a flexible interface to the

ADC The output common-mode voltage of the AD8138 is

easily set to one-half of AVDD, and the driver can be configured

in a Sallen-Key filter topology to provide band limiting of the

input signal.

At input frequencies above Nyquist, the performance of most

amplifiers is not adequate to achieve the true performance of

the AD6652 ADC stage.

r T1 is a center-tapped, 1:4 impedance

mer. The signal characteristics must

s within the allowable range for the selected referenc

The minimum and maximum common- ode in

levels are defined as follows:

VCMMIN = VREF/2

VCMMAX = (AVDD + VREF)/2

The minimum common-mode input level allows the AD66

accommodate ground-referenced inputs.

Although optimum performance is achieved with a differ

This is especially true in IF undersampling applications in

which input frequencies in the range of 70 MHz to 200 MHz are

being sampled. For these applications, differential transform

coupling is the recommended input configuration, as shown i

Figure 39. Transforme

ratio broadband RF transfor

be considered when selecting a transformer. Most RF

transformers saturate at frequencies below a few MHz, and

excessive signal power can also cause core saturation, which

leads to distortion.

AD6652

VINA

AVDD

VINB

AGND

1V p-p

Ω

10pF

49.9

Ω

10pF

Ω

0.1

µF

Ω

Coupled Input for One Channel of the AD6652

1/2 (AVDD + VREF)

REFB = 1/2 (AVDD − VREF)

Span = 2 × (REFT − REFB) = 2 × VREF

As shown by the equations above, the REFT and REFB voltages

are symmetrical about the midsupply voltage and, by definition,

the input span is twice the value of the VREF voltage. Proper

operation of the AD6652 requires that VREF be no less than

0.5 V and no greater than 1.0 V.

The internal voltage reference can be pin-strapped to fixed

values of 0.5 V or 1.0 V, or adjusted within the same range, as

discussed in the Internal Reference Connection section. Maxi-

mum SNR performance is achieved with the reference set to the

largest input span of 2 V p-p. The relative SNR degradation is

3 dB when changing from 2 V p-p mode to 1 V p-p mode.

If operation using an external reference voltage is desired, it can

be substituted for the internal reference, as detailed in the

External Reference Operation section.

Figure 39. Differential AC-

ADC Voltage Reference

A stable and accurate 0.5 V voltage reference is built into the

AD6652. The input span of the ADC tracks reference voltage

changes linearly. An internal differential reference buffer creates

positive and negative reference voltages, REFT and REFB,

respectively, that define the span of the ADC core. The output

common mode of the reference buffer is set to midsupply, and

the REFT and REFB voltages and span are defined as follows:

REFT =

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