AD5425_05 Просмотр технического описания (PDF) - Analog Devices

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AD5425_05

8-Bit, High Bandwidth Multiplying DAC with Serial Interface

Analog Devices 

AD5425

R3

20kΩ

VDD

R2

R5

20kΩ

VREF

±10V

VDD

RFB

IOUT1

R1

VREF

AD5425

IOUT2

SYNC SCLK SDIN GND

MICROCONTROLLER

C1

AA1 1

AGND

R4

10kΩ

A2

VOUT = –VREF

TO +VREF

NOTES:

1. R1 AND R2 ARE USED ONLY IF GAIN ADJUSTMENT IS REQUIRED. ADJUST R1 FOR

VOUT = 0 V WITH CODE 10000000 LOADED TO DAC.

2. MATCHING AND TRACKING IS ESSENTIAL FOR RESISTOR PAIRS R3 AND R4.

3. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED IF A1/A2 IS A HIGH

SPEED AMPLIFIER.

Bipolar Operation

Figure 31. Bipolar Operation (4-Quadrant Multiplication)

Stability

In some applications, it may be necessary to generate full 4-

quadrant multiplying operation or a bipolar output swing. This

can be easily accomplished by using another external amplifier

and some external resistors, as shown in Figure 31. In this

circuit, the second amplifier, A2, provides a gain of 2. Biasing

the external amplifier with an offset from the reference voltage,

results in full 4-quadrant multiplying operation. The transfer

function of this circuit shows that both negative and positive

output voltages are created as the input data, D, is incremented

from code zero (VOUT = −VREF) to midscale (VOUT = 0 V ) to full

scale (VOUT = +VREF).

( ) VOUT = VREF × D /2n−1 −VREF

Where D is the fractional representation of the digital word

loaded to the DAC and n is the resolution of the DAC.

In the I-to-V configuration, the IOUT of the DAC and the

inverting node of the op amp must be connected as closely as

possible and proper PCB layout techniques must be employed.

Since every code change corresponds to a step function, gain

peaking can occur if the op amp has limited GBP and there is

excessive parasitic capacitance at the inverting node. This

parasitic capacitance introduces a pole into the open-loop

response, which can cause ringing or instability in closed-loop

applications.

An optional compensation capacitor, C1, can be added in

parallel with RFB for stability, as shown in Figure 30 and Figure

31. Too small a value of C1 can produce ringing at

the output, while too large a value can adversely affect the

settling time. C1 should be found empirically, but 1 pF to

2 pF is generally adequate for compensation.

When VIN is an ac signal, the circuit performs 4-quadrant

multiplication.

Table 6 shows the relationship between digital code and the

expected output voltage for bipolar operation.

Table 6. Bipolar Code Table

Digital Input

Analog Output (V)

1111 1111

+VREF (127/128)

1000 0000

0

0000 0001

−VREF (127/128)

0000 0000

−VREF (128/128)

Rev. A | Page 15 of 28

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