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AD9238
(Rev.:RevA)

12-Bit, 20/40/65 MSPS Dual A/D Converter

Analog Devices 

AD9238

H

T

VIN+

CPAR

T

5pF

T

VIN–

CPAR

5pF

T

H

Figure 5. Switched Capacitor Input

An internal differential reference buffer creates positive and nega-

tive 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 = 1/2 AVDD +VREF

( ) REFB = 1/2 AVDD −VREF

( ) Span = 2 × REFT − REFB = 2 ×VREF

It can be seen from the equations above that 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.

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. Maximum SNR

performance will be achieved with the AD9238 set to the largest

input span of 2 V p-p. The relative SNR degradation will be 3 dB

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

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

peaks within the allowable range for the selected reference volt-

age. The minimum and maximum common-mode input levels

are defined as follows:

The output common-mode voltage of the AD8138 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.

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

performance of most amplifiers will not be adequate to achieve

the true performance of the AD9238. This is especially true in

IF undersampling applications where frequencies in the 70 MHz

to 200 MHz range are being sampled. For these applications,

differential transformer coupling is the recommended input con-

figuration, as shown in Figure 6.

2V p-p

49.9

0.1F

50

10pF

50

10pF

1k

1k

AVDD

VINA

AD9238

VINB

AGND

Figure 6. Differential Transformer Coupling

The signal characteristics must be considered when selecting a

transformer. Most RF transformers will saturate at frequencies

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

saturation, which leads to distortion.

Single-Ended Input Configuration

A single-ended input may provide adequate performance in

cost-sensitive applications. In this configuration, there will be a

degradation in SFDR and in distortion performance due to the

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

ances on each input are matched, there should be little effect on

SNR performance.

CLOCK INPUT AND CONSIDERATIONS

Typical high speed ADCs use both clock edges to generate a

variety of internal timing signals, and as a result may be sensitive

to clock duty cycle. Commonly, a 5% tolerance is required on the

clock duty cycle to maintain dynamic performance characteristics.

VCMMIN = VREF 2

VCMMAX = (AVDD +VREF ) 2

The minimum common-mode input level allows the AD9238

to accommodate ground-referenced inputs. Although optimum

performance is achieved with a differential input, a single-ended

source may be driven into VIN+ or VIN–. In this configuration,

one input will accept 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 may be applied to VIN+ while a

1 V reference is applied to VIN–. The AD9238 will then accept

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 will be

less noticeable at lower input frequencies and in the lower speed

grade models (AD9238-40 and AD9238-20).

Differential Input Configurations

As previously detailed, optimum performance will be achieved

while driving the AD9238 in a differential input configuration.

For baseband applications, the AD8138 differential driver pro-

vides excellent performance and a flexible interface to the ADC.

The AD9238 provides separate clock inputs for each channel. The

optimum performance is achieved with the clocks operated at the

same frequency and phase. Clocking the channels asynchronously

may degrade performance significantly. In some applications, it is

desirable to skew the clock timing of adjacent channels. The AD9238’s

separate clock inputs allow for clock timing skew (typically ±1 ns)

between the channels without significant performance degradation.

The AD9238-65 contains two clock duty cycle stabilizers, one for

each converter, that retime the nonsampling edge, providing an

internal clock with a nominal 50% duty cycle (DCS is not avail-

able on the –40 MSPS or –20 MSPS versions). Input clock rates

of over 40 MHz can use the DCS so that a wide range of input

clock duty cycles can be accommodated. Maintaining a 50% duty

cycle clock is particularly important in high speed applications,

when proper track-and-hold times for the converter are required

to maintain high performance. The DCS can be enabled by tying

the DCS pin high.

The duty cycle stabilizer utilizes a delay locked loop to create the

nonsampling edge. As a result, any changes to the sampling fre-

quency will require approximately 2 µs to 3 µs to allow the DLL

to acquire and settle to the new rate.

REV. A

–13–

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