AD9212 datasheet(21/56 Pages) AD

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AD9212

Rev. 0 | Page 21 of 56

For best dynamic performance, the source impedances driving

VIN+ and VIN− should be matched such that common-mode

settling errors are symmetrical. These errors are reduced by the

common-mode rejection of the ADC. An internal reference

buffer creates the 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

REFT = 1/2 (AVDD + VREF)

REFB = 1/2 (AVDD − VREF)

Span = 2 × (REFT − REFB) = 2 × VREF

It can be seen from these equations 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.

Maximum SNR performance is always achieved by setting the

ADC to the largest span in a differential configuration. In the

case of the AD9212, the largest input span available is 2 V p-p.

Differential Input Configurations

There are several ways in which to drive the AD9212 either

actively or passively. In either case, the optimum performance is

achieved by driving the analog input differentially. One example

is by using the AD8334 differential driver. It provides excellent

performance and a flexible interface to the ADC (see Figure 50)

for baseband applications. This configuration is common for

medical ultrasound systems.

However, the noise performance of most amplifiers is not

adequate to achieve the true performance of the AD9212. For

applications where SNR is a key parameter, differential transfor-

mer coupling is the recommended input configuration. Two

examples are shown in Figure 47 and Figure 48.

In any configuration, the value of the shunt capacitor, C, is

dependent on the input frequency and may need to be reduced

or removed.

2V p-p

CDIFF1

1CDIFF IS OPTIONAL.

49.9Ω

0.1μF

1kΩ

AGND

AVDD

ADT1–1WT

1:1 Z RATIO

VIN–

ADC

AD9212

VIN+

Figure 47. Differential Transformer-Coupled Configuration

for Baseband Applications

ADC

AD9212

2V p-p

2.2pF

1kΩ

0.1μF

1kΩ

AVDD

ADT1–1WT

1:1 Z RATIO

16nH

0.1μF

16nH

33Ω

499Ω

65Ω

VIN+

VIN–

Figure 48. Differential Transformer-Coupled Configuration for IF Applications

Single-Ended Input Configuration

A single-ended input may provide adequate performance in

cost-sensitive applications. In this configuration, SFDR and

distortion performance degrade due to the large input common-

mode swing. If the application requires a single-ended input

configuration, ensure that the source impedances on each input

are well matched in order to achieve the best possible performance.

A full-scale input of 2 V p-p can still be applied to the ADC’s VIN+

pin while the VIN− pin is terminated. Figure 49 details a typical

single-ended input configuration.

2V p-p

49.9Ω

0.1µF

AVDD

1kΩ 25Ω

1kΩ

AVDD

VIN–

ADC

AD9212

VIN+

CDIFF1

1CDIFF IS OPTIONAL.

Figure 49. Single-Ended Input Configuration

AD8334

1.0kΩ

374Ω

187Ω

0.1μF

187Ω

0.1μF

0.1μF10μF

0.1μF

1V p-p

0.1μF

LNA

120nH

VGA

VOH

VIP

INH

22pF

LMD

VIN

LOP

LON

VOL

18nF

274Ω

VIN–

ADC

AD9212

VIN+

VREF

Figure 50. Differential Input Configuration Using the AD8334

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