AD9212-65EBZ datasheet(23/57 Pages) AD

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AD9212

Data Sheet

Rev. E | Page 22 of 56

For best dynamic performance, the source impedances driving

VIN + x and VIN − x 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 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 to drive the AD9212 either actively or

passively; however, optimum performance is achieved by driving

the analog input differentially. For example, using the AD8334

differential driver to drive the AD9212 provides excellent perfor-

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

baseband applications. This configuration is commonly used

for medical ultrasound systems.

For applications where SNR is a key parameter, differential

transformer coupling is the recommended input configuration

(see Figure 47 and Figure 48), because the noise performance of

most amplifiers is not adequate to achieve the true performance

of the AD9212.

Regardless of the 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 – x

ADC

AD9212

VIN + x

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+x

VIN– x

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 + x pin while the VIN − x 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 – x

ADC

AD9212

VIN + x

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 – x

ADC

AD9212

VIN + x

1kΩ

AVDD

Figure 50. Differential Input Configuration Using the AD8334

AD9212-65EBZ Datasheet(PDF) 23 Page - Analog Devices

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