AD9260AS datasheet(28/45 Pages) AD

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AD9260

Rev. C | Page 27 of 44

VIN

100pF

VCML-VIN

Ω

VCML-VIN

0.1

µF

1.0

µF

AD817

AD9260

Ω

VINA

VINB

CML

100pF

Figure 64. DC-Coupled Differential Driver with Level-Shifting

beyond the input signals passband by adding a shunt capacitor,

CF, across the feedback resistor of each op amp. This will

essentially establish a low-pass filter which reduces the noise

gain to one beyond the filter’s f–3 dB while simultaneously

bandlimiting the input signal to f–3 dB. Note that the pole

established by this filter can also be used as the real pole of an

antialiasing filter. Since the noise contribution of two op amps

from the same product family are typically equal but

uncorrelated, the total output-referred noise of each op amp

will add root-sum square leading to a further 3 dB degradation

in the circuit’s noise performance. Further out-of-band noise

reduction can be realized with the addition of single-ended and

differential capacitors, CS and CD.

The distortion and noise performance of the two op amps

within the signal path are critical in achieving optimum

performance in the AD9260. Low noise op amps capable of

providing greater than 85 dB THD at 1 MHz while swinging

over a 1 V to 3 V range are a rare commodity, yet these parts are

the only ones that should be considered. The AD9632 op amp

was found to provide superb distortion performance in this

circuit due to its ability to maintain excellent distortion

performance over a wide bandwidth while swinging over a 1 V

to 3 V range. Since the AD9632 is gain-of-two or greater stable,

the use of the noise reduction shunt capacitors discussed above

was prohibited, thus degrading its noise performance slightly

(1 dB–2 dB) when compared to the OPA642. Note that the

majority of the AD9260 test and characterization data presented

in this data sheet was taken using the AD9632 op amp in this

dc-coupled driver circuit. This driver circuit is also provided on

the AD9260 evaluation board since the AD8138 was unreleased

at that time.

The outputs of each op amp are ac coupled via a small series

resistor and capacitor (i.e., 50 Ω and 0.1 µF) to the respective

inputs of the AD9260. Similar to the dc coupled driver, further

out-of-band noise reduction can be realized with the addition of

100 pF single-ended and differential capacitors, CS and CD. The

lower cutoff frequency of this ac-coupled circuit is determined

by RC and CC in which RC is tied to the common-mode level pin,

CML, of the AD9260 for proper biasing of the inputs. Although

the OPA642 was found to provide the lowest overall noise and

distortion performance (88.8 dB and 96 dB THD @ 100 kHz),

the AD8055, or dual AD8056, suffered only a 0.5 dB to 1.5 dB

degradation in overall performance. It is worth noting that

given the high level of performance attainable by the AD9260,

special consideration must be given to both the quality of the

test equipment and test set-up in its evaluation.

Common-Mode Level

The CML pin is an internal analog bias point used internally by

the AD9260. This pin must be decoupled to analog ground with

at least a 0.1 µF capacitor as shown in Figure 65. The dc level of

CML is approximately AVDD/2.5. This voltage should be

buffered if it is to be used for any external biasing.

Note: the common-mode voltage of the input signal applied to

the AD9260 need not be at the exact same level as CML. While

this level is recommended for optimal performance, the

AD9260 is tolerant of a range of input common-mode voltages

around AVDD/2.5.

CML

AD9260

0.1

µF

Figure 65. CML Decoupling

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