AD7948ARS-B datasheet(14/16 Pages) AD

14 / 16 page

AD7943/AD7945/AD7948

REV. B

–14–

BIPOLAR OPERATION

(Four-Quadrant Multiplication)

Figure 15 shows the standard connection diagram for bipolar

operation of the AD7943, AD7945 and AD7948. The coding is

offset binary as shown in Table IV. When VIN is an ac signal,

the circuit performs four-quadrant multiplication. Resistors R1

and R2 are for gain error adjustment and are not needed in

many applications where the device gain error specifications are

adequate. To maintain the gain error specifications, resistors

R3, R4 and R5 should be ratio matched to 0.01%.

IOUT1

IOUT2

VOUT

SIGNAL GROUND

AGND

DAC

VREF

R1 20

AD7943/45/48

VIN

R2 10

RFB

R4 20k

10k

20k

NOTES

1. ONLY ONE DAC IS SHOWN FOR CLAIRITY.

2. DIGITAL INPUT CONNECTIONS ARE OMITTED.

3. C1 PHASE COMPENSATION (5 – 15pF) MAY BE REQUIRED

WHEN USING HIGH SPEED AMPLIFIER, A1.

Figure 15. Bipolar Operation (Four-Quadrant

Multiplication)

Suitable dual amplifiers for use with Figure 15 are the OP270

(low noise, low bandwidth, 15 kHz), the AD712 (medium

bandwidth, 200 kHz) or the AD827 (wide bandwidth, 1 MHz).

Table IV. Bipolar (Offset Binary) Code

Table Digital Input

Analog Output

MSB

LSB

(VOUT as Shown in Figure 15)

1111 1111 1111

+VREF (2047/2048)

1000 0000 0001

+VREF (1/2048)

1000 0000 0000

+VREF (0/2048) = 0

0111 1111 1111

–VREF (1/2048)

0000 0000 0001

–VREF (2047/2048)

0000 0000 0000

–VREF (2048/2048) = –VREF

NOTE

Nominal LSB size for the circuit of Figure 15 is given by: V REF (1/2048).

SINGLE SUPPLY APPLICATIONS

The “-B” versions of the devices are specified and tested for

single supply applications. Figure 16 shows the recommended

circuit for operation with a single +5 V to +3.3 V supply. The

IOUT2 and AGND terminals are biased to 1.23 V. Thus, with 0 V

applied to the VREF terminal, the output will go from 1.23 V (all

0s loaded to the DAC) to 2.46 V (all 1s loaded). With 2.45 V

applied to the VREF terminal, the output will go from 1.23 V (all

0s loaded) to 0.01 V (all 1s loaded). It is important when con-

sidering INL in a single-supply system to realize that most

single-supply amplifiers cannot sink current and maintain zero

volts at the output. In Figure 16, with VREF = 2.45 V the re-

quired sink current is 200

µA. The minimum output voltage

level is 10 mV. Op amps like the OP295 are capable of main-

taining this level while sinking 200

µA.

Figure 16 shows the IOUT2 and AGND terminals being driven

by an amplifier. This is to maintain the bias voltage at 1.23 V

as the impedance seen looking into the IOUT2 terminal changes.

This impedance is code dependent and varies from infinity (all

0s loaded in the DAC) to about 6 k

Ω minimum. The AD589

has a typical output resistance of 0.6

Ω and it can be used to

drive the terminals directly. However, this will cause a typical

linearity degradation of 0.2 LSBs. If this is unacceptable then

the buffer amplifier is necessary. Figure 9 shows the typical

linearity performance of the AD7943/AD7945/AD7948 when

used as in Figure 16 with VDD set at +3.3 V and VREF = 0 V.

IOUT1

IOUT2

VOUT

SIGNAL GROUND

A1: OP295

AD822

OP283

AGND

DAC

VREF

AD7943/45/48

VIN

RFB

+5V

5.6k

AD589

+3.3V

VDD

DGND

Figure 16. Single Supply System

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