AD9225ARS datasheet(13/24 Pages) AD

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AD9225

–13–

REV. A

10 F

VINA

VINB

SENSE

AD9225

0.1 F

–V

VREF

2.0V

Figure 19. Single-Ended AD9225 Op Amp Drive Circuit

Op Amp with DC Level Shifting

Figure 20 shows a dc-coupled level shifting circuit employing an

op amp, A1, to sum the input signal with the desired dc set.

Configuring the op amp in the inverting mode with the given

resistor values results in an ac signal gain of –1. If the signal

inversion is undesirable, interchange the VINA and VINB con-

nections to reestablish the original signal polarity. The dc volt-

age at VREF sets the common-mode voltage of the AD9225.

For example, when VREF = 2.0 V, the input level from the op

amp will also be centered around 2.0 V. The use of ratio matched,

thin-film resistor networks will minimize gain and offset errors.

Also, an optional pull-up resistor, RP, may be used to reduce

the output load on VREF to less than its 1 mA maximum.

0VDC

+VREF

–VREF

VINA

VINB

AD9225

0.1 F

500 *

0.1 F

500 *

+VCC

500 *

VREF

500 *

RP**

*OPTIONAL RESISTOR NETWORK-OHMTEK ORNA500D

**OPTIONAL PULL-UP RESISTOR WHEN USING INTERNAL REFERENCE

Figure 20. Single-Ended Input with DC-Coupled Level Shift

AC COUPLING AND INTERFACE ISSUES

For applications where ac coupling is appropriate, the op amp’s

output can be easily level-shifted via a coupling capacitor. This

has the advantage of allowing the op amp’s common-mode level

to be symmetrically biased to its midsupply level (i.e., (VCC +

VEE)/2). Op amps that operate symmetrically with respect to

their power supplies typically provide the best ac performance as

well as greatest input/output span. Various high speed/perfor-

mance amplifiers which are restricted to +5 V/–5 V operation

and/or specified for +5 V single-supply operation can be easily

configured for the 4 V or 2 V input span of the AD9225. Note

that differential transformer coupling, which is another form of

ac coupling, should be considered for optimum ac performance.

Simple AC Interface

Figure 21 shows a typical example of an ac-coupled, single-

ended configuration. The bias voltage shifts the bipolar, ground-

referenced input signal to approximately AVDD/2. The value

for C1 and C2 will depend on the size of the resistor, R. The

capacitors, C1 and C2, are a 0.1

µF ceramic and 10 µF tanta-

lum capacitor in parallel to achieve a low cutoff frequency while

maintaining a low impedance over a wide frequency range. The

combination of the capacitor and the resistor form a high-pass

filter with a high-pass –3 dB frequency determined by the equation,

f–3 dB = 1/(2

× π × R × (C1 + C2))

The low impedance VREF voltage source biases both the VINB

input and provides the bias voltage for the VINA input. Figure

21 shows the VREF configured for 2.0 V thus the input range of

the A/D is 0 V to 4 V. Other input ranges could be selected by

changing VREF.

VINA

VINB

AD9225

+5V

–5V

+2V

–2V

VIN

10 F

AD9631

0.1 F

10 F

0.1 F

0.5

2.5

4.5

Figure 21. AC-Coupled Input

Alternative AC Interface

Figure 22 shows a flexible ac coupled circuit which can be con-

figured for different input spans. Since the common mode volt-

age of VINA and VINB are biased to midsupply independent of

VREF, VREF can be pin strapped or reconfigured to achieve

input spans between 2 V and 4 V p-p. The AD9225’s CMRR

along with the symmetrical coupling R-C networks will reject

both power supply variations and noise. The resistors, R, estab-

lish the common-mode voltage. They may have a high value

(e.g., 5 k

Ω) to minimize power consumption and establish a low

cutoff frequency. The capacitors, C1 and C2, are typically a

0.1

µF ceramic and 10 µF tantalum capacitor in parallel to

achieve a low cutoff frequency while maintaining a low imped-

ance over a wide frequency range. RS isolates the buffer ampli-

fier from the A/D input. The optimum performance is achieved

when VINA and VINB are driven via symmetrical networks.

The f–3 dB point can be approximated by the equation,

f –3dB

π×6K +(C1+ C2)

VINA

VINB

AD9225

VIN

0.1 F

VCM

0.1 F

10 F

0.1 F

Figure 22. AC-Coupled Input-Flexible Input Span,

VCM = 2.5 V

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