AD9220ARSZ datasheet(23/32 Pages) AD

23 / 32 page

REV. E

AD9221/AD9223/AD9220

–23–

APPLICATIONS

Direct IF Down Conversion Using the AD9220

As previously noted, the AD9220’s performance in the differen-

tial mode of operation extends well beyond its baseband region

and into several Nyquist zone regions. Thus, the AD9220 may

be well suited as a mix down converter in both narrow and

wideband applications. Various IF frequencies exist over the

frequency range in which the AD9220 maintains excellent

dynamic performance (e.g., refer to Figure 17 and 18). The IF

signal will be aliased to the ADC’s baseband region due to the

sampling process in a similar manner that a mixer will down-

convert an IF signal. For signals in various Nyquist zones, the

following equation may be used to determine the final frequency

after aliasing.

f1 NYQUIST = fSIGNAL

f2 NYQUIST = fSAMPLE – fSIGNAL

f3 NYQUIST = abs (fSAMPLE – fSIGNAL)

f4 NYQUIST = 2

× f

SAMPLE – fSIGNAL

f5 NYQUIST = abs (2

× f

SAMPLE – fSIGNAL)

There are several potential benefits in using the ADC to alias

(i.e., or mix) down a narrow-band or wideband IF signal. First

and foremost is the elimination of a complete mixer stage with

its associated amplifiers and filters, reducing cost and power

dissipation. Second is the ability to apply various DSP tech-

niques to perform such functions as filtering, channel selection,

quadrature demodulation, data reduction, and detection.

One common example is the digitization of a 21.4 MHz IF

using a low jitter 10 MHz sample clock. Using the equation

above for the fifth Nyquist zone, the resultant frequency after

sampling is 1.4 MHz. Figure 33 shows the typical performance

of the AD9220 operating under these conditions. Figure 34

demonstrates how the AD9220 is still able to maintain a high

degree of linearity and SFDR over a wide amplitude.

FREQUENCY – MHz

–120

–40

–60

–80

–20

–100

ENCODE = 10MSPS

AIN = 21.4MHz

Figure 33. IF Sampling a 21.4 MHz Input Using

the AD9220 (VCM = 2.5 V, Input Span = 2 V p-p)

AIN – dB

–50

–40

–30

–20

–10

SFDR

SNR

Figure 34. AD9220 Differential Input SNR/SFDR

vs. Input Amplitude (AIN) @ 21.4 MHz

Multichannel Data Acquisition with Autocalibration

The AD9221/AD9223/AD9220 is well suited for high perfor-

mance, low power data acquisition systems. Aside from its

exceptional ac performance, it exhibits true 12-bit linearity and

temperature drift performance (i.e., excluding internal refer-

ence). Furthermore, the A/D product family provides the system

designer with an upward or downward component selection

path based on power consumption and sampling rate.

A typical multichannel data acquisition system is shown in

Figure 35. Also shown is some additional inexpensive gain and

offset autocalibration circuitry that is often required in high

accuracy data acquisition systems. These additional peripheral

components were selected based on their performance, power

consumption, and cost.

Referring to Figure 35, the AD9221/AD9223/AD9220 is config-

ured for single-ended operation with a 2.5 V p-p input span and

a 2.5 V common-mode voltage using an external, precision 2.5

voltage reference, U1. This configuration and input span allows

the buffer amplifier, U4, to be single supply. Also, it simplifies

the design of the low temperature drift autocalibration circuitry

that uses thin-film resistors for temperature stability and ratio-

metric accuracy. The input of the AD9221/AD9223/AD9220

can be easily configured for a wider span but it should remain

within the input/output swing capabilities of a high speed, rail-

to-rail, single-supply amplifier, U4 (e.g., AD8041).

The gain and offset calibration circuitry is based on two 8-bit,

current-output DAC08s, U3 and U5. The gain calibration

circuitry consisting of U3, and an op amp, U2A, is configured

to provide a low drift nominal 1.25 V reference to the AD9221/

AD9223/AD9220. The resistor values that set the gain calibra-

tion range were selected to provide a nominal adjustment span

±128 LSBs with 1 LSB resolution with respect to the A/D. Note

that the bandwidth of the reference is low and, as a result, it is

not possible to change the reference voltage rapidly in this mode.

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