In differential applications, the center point of the span isobtained by the common-mode level of the signals. In single-ended applications, the center point is the dc potential appliedto one input pin while the signal is applied to the opposite inputpin. Figures 5a–5f show various system configurations.
DRIVING THE ANALOG INPUTS
The AD9226 has a very flexible input structure allowing it tointerface with single-ended or differential input interface circuitry.The optimum mode of operation, analog input range, and asso-ciated interface circuitry will be determined by the particularapplications performance requirements as well as power supplyoptions.
DIFFERENTIAL DRIVER CIRCUITS
Differential operation requires that VINA and VINB be simulta-neously driven with two equal signals that are 180 out of phasewith each other.
Differential modes of operation (ac- or dc-coupled input) providethe best THD and SFDR performance over a wide frequencyrange. They should be considered for the most demanding
spectral-based applications (e.g., direct IF conversion to digital).
REV. 0
–14–
Figure 5a.1 V Single-Ended Input, Common-ModeVoltage = 1V
Figure 5b.1V Differential Input, Common-Mode
Voltage = 1 V
Figure 5c.2 V Differential Input, Common-ModeVoltage = 2 V
Figure 5d.2 V Single-Ended Input, Common-ModeVoltage = 2 V
REV. 0
AD9226
Figure 5e.2 V Differential Input, Common-ModeVoltage = 2.5 V
Figure 5f.1 V Differential Input, Common-Mode
Voltage = 2.5 V (Recommended for IF Undersampling)
The differential input characterization for this data sheet wasperformed using the configuration shown in Figure 7.
Since not all applications have a signal preconditioned fordifferential operation, there is often a need to perform a single-ended-to-differential conversion. In systems that do not need tobe dc-coupled, an RF transformer with a center tap is the bestmethod to generate differential inputs for the AD9226. It pro-vides all the benefits of operating the ADC in the differentialmode without contributing additional noise or distortion. An RFtransformer also has the added benefit of providing electricalisolation between the signal source and the ADC. An improvementin THD and SFDR performance can be realized by operatingthe AD9226 in the differential mode. The performance enhance-ment between the differential and single-ended mode is mostnoteworthy as the input frequency approaches and goes beyondthe Nyquist frequency (i.e., fIN > FS /2).
The circuit shown in Figure 6a is an ideal method of applyinga differential dc drive to the AD9226. It uses an AD8138 toderive a differential signal from a single-ended one. Figure 6billustrates its performance.
Figure 7 presents the schematic of the suggested transformercircuit. The circuit uses a Minicircuits RF transformer, modelT1-1T, which has an impedance ratio of four (turns ratio of 2).The schematic assumes that the signal source has a 50 sourceimpedance. The center tap of the transformer provides a con-venient means of level-shifting the input signal to a desiredcommon-mode voltage. In Figure 7 the transformer centertapis connected to a resistor divider at the midsupply voltage.–15–
AD9226
SINGLE-ENDED DRIVER CIRCUITS
F
F
F
The AD9226 can be configured for single-ended operation usingdc- or ac-coupling. In either case, the input of the ADC must bedriven from an operational amplifier that will not degrade theADC’s performance. Because the ADC operates from a singlesupply, it will be necessary to level-shift ground-based bipolarsignals to comply with its input requirements. Both dc- andac-coupling provide this necessary function, but each methodresults in different interface issues which may influence thesystem design and performance.
Single-ended operation requires that VINA be ac- or dc-coupledto the input signal source, while VINB of the AD9226 be biasedto the appropriate voltage corresponding to the middle of the inputspan. The single-ended specifications for the AD9226 are char-acterized using Figure 9a circuitry with input spans of 1 V and2V. The common-mode level is 2.5 V.
If the analog inputs exceed the supply limits, internal parasiticdiodes will turn on. This will result in transient currents withinthe device. Figure 8 shows a simple means of clamping an input.It uses a series resistor and two diodes. An optional capacitor isshown for ac-coupled applications. A larger series resistor canbe used to limit the fault current through D1 and D2. Thiscan cause a degradation in overall performance. A similarclamping circuit can also be used for each input if a differen-tial input signal is being applied. A better method to ensurethe input is not overdriven is to use amplifiers powered by a single5 V supply such as the AD8138.
Figure 6a.Direct-Coupled Drive Circuit with AD8138Differential Op Amp
––dBc
––––0
4
8
12
16MHz
20
24
28
32
Figure 6b.FS = 65 MSPS, fIN = 30 MHz, Input Span = 1 V p-p
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