AD9712B/AD9713B

REV. B

–6–

The REFERENCE IN pin can also be driven directly for wider

bandwidth multiplying operation. The analog signal for this

mode of operation must have a signal swing in the range of

–3.75 V to –4.25 V. This can be implemented by capacitively

coupling into REFERENCE IN a signal with a dc bias of –3.75 V

to –4.25 V, as shown in Figure 3; or by driving REFERENCE

IN with a low impedance op amp whose signal swing is limited

to the stated range.

Outputs

15 discrete current sinks. D5 and D6 are binarily weighted; and

tecture reduces frequency domain errors due to glitch impulse.

REFERENCE

IN

AD9712B

AD9713B

–

17

Figure 3. Wideband Multiplying Circuit

The Switch Network provides complementary current outputs

current source matching which provides 12-bit linearity without

the digital input code. The sum of the two currents is always

equal to the full-scale output current minus one LSB.

The current output can be converted to a voltage by resistive

loaded equally for best overall performance. The voltage which

is developed is the product of the output current and the value

of the load resistor.

Full-scale output current is determined by CONTROL AMP

× 128

The internal reference is nominally –1.18 V with a tolerance of

±3.5% and typical drift over temperature of 50 ppm/°C. If

greater accuracy or better temperature stability is required, an

external reference can be utilized. The AD589 reference shown

in Figure 1 features

±10 ppm/°C drift over temperatures from

0

°C to +70°C.

CONTROL

AMP IN

AD9712B

AD9713B

AD589

+

–

~

–11k

19

Figure 1. Use of AD589 as External Reference

Two modes of multiplying operation are possible with the

AD9712B/AD9713B. Signals with small signal bandwidths up

to 300 kHz and input swings of 100 mV, or dc signals from

–0.6 V to –1.2 V can be applied to the CONTROL AMP input

as shown in Figure 2. Because the control amplifier is internally

compensated, the 0.1

µF capacitor at Pin 17 can be reduced to

0.01

µF to maximize the multiplying bandwidth. However, it

should be noted that settling time for changes to the digital in-

puts will be degraded.

CONTROL

AMP IN

AD9712B

AD9713B

19

24

17

18

18

REFERENCE

IN

CONTROL

AMP OUT

–0.6V TO –1.2V

300 kHz MAX

Figure 2. Low Frequency Multiplying Circuit