Datasheet AD9241 (Analog Devices) - 14

AD9241 DC Coupling with Op Amps 390
Ω Applications that require dc coupling can also benefit by driv-
AVDD
ing the AD9241 differentially. Since the signal swing require-
0.1µF V AD8047 220.2
Ω
CML–VIN
ments of each input is reduced by a factor of two in the differential
390
Ω
33
Ω mode, the AD9241 can be configured for a 5 V input span in a
VINA
+5 V or ± 5 V system. This allows various high performance op
390
Ω
390
Ω
VIN
amps specified for +5 V and ± 5 V operation to be configured in
AVDD
various differential driver topologies. The optimum op amp
220.2
Ω
390
Ω
390
Ω driver topology depends on whether the common-mode voltage
AD9241 0.1µF
of the single-ended-input signal requires level-shifting.
V AD8047 CML+VIN 33
Ω Figure 30 shows a cross-coupled differential driver circuit best
VINB
suited for systems in which the common-mode signal of the
390
Ω
390
Ω
2.5k
Ω input is already biased to approximately midsupply (i.e., 2.5 V).
0.1µF
The common-mode voltage of the differential output is set by
100
Ω the voltage applied to the “+” input of A2. The closed loop
CML
gain of this symmetrical driver can easily be set by RIN and RF.
0.1µF 1µF OP113
For more insight into the operation of this cross-coupled driver, please refer to the AD8042 data sheet. Figure 31. Differential Driver with Level-Shifting
1k
Ω
RF VCML+VIN SINGLE-ENDED MODE OF OPERATION 1k
Ω
AD8042 VIN
The AD9241 can be configured for single-ended operation
R VINA IN A1 33
Ω using dc or ac coupling. In either case, the input of the A/D
1k
Ω must be driven from an operational amplifier that will not de- grade the A/D’s performance. Because the A/D operates from a
1k
Ω
1k
Ω
CF*
single supply, it will be necessary to level-shift ground-based
AD9241
bipolar signals to comply with its input requirements. Both dc
V
and ac coupling provide this necessary function, but each
1k
Ω
CML–VIN 33
Ω method results in different interface issues that may influence
A2 VINB
the system design and performance.
AD8042 AVDD/2 CML DC COUPLING AND INTERFACE ISSUES 0.1µF
Many applications require the analog input signal to be dc
*OPTIONAL NOISE/BAND LIMITING CAPACITOR
coupled to the AD9241. An operational amplifier can be con- figured to rescale and level-shift the input signal to make it Figure 30. Cross-Coupled Differential Driver compatible with the selected input range of the A/D. The input The driver circuit shown in Figure 31 is best suited for systems range to the A/D should be selected on the basis of system in which the bipolar input signal is referenced to AGND and performance objectives as well as the analog power supply requires proper level shifting. This driver circuit provides the availability since this will place certain constraints on the op ability to level-shift the input signal to within the common- amp selection. mode range of the AD9241. The two op amps are configured as Many of the new high performance op amps are specified for matched difference amplifiers, with the input signal applied to only ± 5 V operation and have limited input/output swing capa- opposing inputs to provide the differential output. The common- bilities. Hence, the selected input range of the AD9241 should mode offset voltage is applied to the noninverting resistor net- be sensitive to the headroom requirements of the particular op work that provides the proper level-shifting. The circuit also amp to prevent clipping of the signal. Also, since the output of employs optional diodes and pull-up resistors that may help a dual supply amplifier can swing below –0.3 V, clamping its improve the op amps’ distortion performance by reducing their output should be considered in some applications. headroom requirements. Rail-to-rail output amplifiers such as In some applications, it may be advantageous to use an op amp the AD8042 have sufficient headroom and do not require these specified for single supply +5 V operation since it will inher- optional components. ently limit its output swing to within the power supply rails. Rail-to-rail output amplifiers such as the AD8041 allow the AD9241 to be configured with larger input spans, which im- proves the noise performance. REV. 0 –13–