Datasheet

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
R
R
C
DIFF
1
C
1
C
DIFF
IS OPTIONAL.
49.9
0.1F
1k
1k
AGND
AVDD
A
DT1-1WT
1:1 Z RATIO
VIN – x
ADC
AD9212
VIN + x
C
05968-018
Figure 47. Differential Transformer-Coupled Configuration
for Baseband Applications
ADC
AD9212
2V p-p
2.2pF
1k
0.1F
1k
1k
AVDD
A
DT1-1WT
1:1 Z RATIO
16nH
16nH
0.1F
16nH
33
33
499
65
VIN+ x
VIN– x
05968-019
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
R
R
49.9
0.1µF
0.1µF
AVDD
1k
25
1k
1k
1k
A
VDD
VIN – x
ADC
AD9212
VIN + x
C
DIFF
1
C
1
C
DIFF
IS OPTIONAL.
C
05968-020
Figure 49. Single-Ended Input Configuration
05968-021
AD8334
1.0k
1.0k
374
187
R
R
C
0.1F
187
0.1
F
0.1F
0.1F
0.1F10F
0.1F
1V p-p
0.1F
LNA
120nH
VGA
VOH
VIP
INH
22pF
LMD
VIN
LOP
LON
VOL
18nF
274
VIN – x
ADC
AD9212
VIN + x
1k
1k
AVDD
Figure 50. Differential Input Configuration Using the AD8334