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AD9244

Rev. C | Page 19 of 36

The optimum noise and dc linearity performance for either

differential or single-ended inputs is achieved with the largest

input signal voltage span (that is, 2 V input span) and matched

input impedance for VIN+ and VIN–. Only a slight degradation

in dc linearity performance exists between the 2 V and 1 V

input spans; however, the SNR is lower in the 1 V input span.

When the ADC is driven by an op amp and a capacitive load is

switched onto the output of the op amp, the output momentar-

ily drops due to its effective output impedance. As the output

recovers, ringing can occur. To remedy the situation, a series

resistor, RS, can be inserted between the op amp and the SHA

input, as shown in Figure 45. A shunt capacitance also acts like

a charge reservoir, sinking or sourcing the additional charge

required by the sampling capacitor, CS, further reducing current

transients seen at the op amp’s output.

VREF

SENSE

AD9244

VIN+

VIN–

20pF

μF

0.1

μF

REFCOM

0.1

μF

VCC

VEE

Figure 45. Resistors Isolating SHA Input from Op Amp

The optimum size of this resistor is dependent on several

factors, including the ADC sampling rate, the selected op amp,

and the particular application. In most applications, a 30 Ω to

100 Ω resistor is sufficient.

For noise-sensitive applications, the very high bandwidth of the

AD9244 can be detrimental, and the addition of a series resistor

and/or shunt capacitor can help limit the wideband noise at the

ADC’s input by forming a low-pass filter. The source impedance

driving VIN+ and VIN− should be matched. Failure to provide

matching can result in degradation of the SNR, THD, and SFDR

performance.

Single-Ended Input Configuration

A single-ended input can provide adequate performance in

cost-sensitive applications. In this configuration, there is

degradation in distortion performance due to large input

common-mode swing. However, if the source impedances on

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

performance.

The internal reference can be used to drive the inputs. Figure 45

shows an example of VREF driving VIN−. In this operating

mode, a 5 Ω resistor and a 0.1 μF capacitor must be connected

between VREF and VIN−, as shown in Figure 45, to limit the

reference noise sampled by the analog input.

Differentially Driving the Analog Inputs

The AD9244 has a very flexible input structure, allowing it to

interface with single-ended or differential inputs.

The optimum mode of operation, analog input range, and associ-

ated interface circuitry is determined by the particular application’s

performance requirements as well as power supply options.

Differential operation requires that VIN+ and VIN− be

simultaneously driven with two equal signals that are 180°out of

phase with each other.

Differential modes of operation (ac-coupled or dc-coupled input)

provide the best SFDR performance over a wide frequency range.

They should be considered for the most demanding spectral-

based applications; that is, direct IF conversion to digital.

Because not all applications have a signal precondition for

differential operation, there is often a need to perform a single-

ended-to-differential conversion. In systems that do not require

dc coupling, an RF transformer with a center tap is the best

method for generating differential input signals for the AD9244.

This provides the benefit of operating the ADC in the differen-

tial mode without contributing additional noise or distortion.

An RF transformer also has the added benefit of providing

electrical isolation between the signal source and the ADC.

The differential input characterization was performed using the

configuration in Figure 46. The circuit uses a Mini-Circuits® RF

transformer, model T1-1T, which has an impedance ratio of 1:1.

This circuit assumes that the signal source has a 50 Ω source

impedance. The secondary center tap of the transformer allows

a dc common-mode voltage to be added to the differential input

signal. In Figure 46, the center tap is connected to a resistor

divider providing a half supply voltage. It could also be

connected to the CML pin of the AD9244. For IF sampling

applications (70 MHz < fIN < 200 MHz), it is recommended that

the 20 pF differential capacitor between VIN+ and VIN− be

reduced or removed.

REFT

REFB

AD9244

VIN+

VIN–

20pF

0.1

μF

0.1

μF

0.1

μF

0.1

μF

AVDD

MINI-CIRCUITS

T1–1T

Figure 46. Transformer-Coupled Input

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