AD5433 datasheet(20/32 Pages) AD

20 / 32 page

AD5424/AD5433/AD5445

Rev. B | Page 20 of 32

ADDING GAIN

As D is reduced, the output voltage increases. For small values

of D, it is important to ensure that the amplifier does not

saturate and that the required accuracy is met.

In applications where the output voltage is required to be

greater than VIN, gain can be added with an additional external

amplifier or it can be achieved in a single stage. It is important

to consider the effect of the temperature coefficients of the thin

film resistors of the DAC. Simply placing a resistor in series

with the RFB resistor causes mismatches in the temperature

coefficients and results in larger gain temperature coefficient

errors. Instead, the circuit shown in Figure 54 is a

recommended method of increasing the gain of the circuit. R1,

R2, and R3 should have similar temperature coefficients, but

they need not match the temperature coefficients of the DAC.

This approach is recommended in circuits where gains greater

than 1 are required.

For example, in the circuit shown in Figure 55, an 8-bit DAC

driven with the binary code 0x10 (00010000), that is,

16 decimal, should cause the output voltage to be 16 × VIN.

However, if the DAC has a linearity specification of ±0.5 LSB,

then D can in fact have a weight anywhere in the range 15.5/256

to 16.5/256 so that the possible output voltage falls in the range

15.5 VIN to 16.5 VIN—an error of 3% even though the DAC itself

has a maximum error of 0.2%.

GND

RFB

VDD

VOUT

VREF

VIN

NOTE:

ADDITIONAL PINS OMITTED FOR CLARITY

IOUT1

IOUT2

8-/10-/12-BIT

DAC

GND

VDD

RFB

VDD

VOUT

VREF

VIN

ADDITIONAL PINS OMITTED FOR CLARITY

C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE

REQUIRED IF A1 IS A HIGH SPEED AMPLIFIER.

NOTES:

IOUT1

IOUT2

R1 =

GAIN =

R2 + R3

R2R3

R2 + R3

Figure 55. Current-Steering DAC Used as a Divider or

Programmable Gain Element

DAC leakage current is also a potential error source in divider

circuits. The leakage current must be counterbalanced by an

opposite current supplied from the op amp through the DAC.

Since only a fraction, D, of the current into the VREF terminal is

routed to the IOUT1 terminal, the output voltage has to change

as follows:

Figure 54. Increasing the Gain of the Current Output DAC

DACS USED AS A DIVIDER OR PROGRAMMABLE

GAIN ELEMENT

Current steering DACs are very flexible and lend themselves to

many different applications. If this type of DAC is connected as

the feedback element of an op amp and RFB is used as the input

resistor, as shown in Figure 55, then the output voltage is

inversely proportional to the digital input fraction, D.

Output Error Voltage due to DAC Leakage = (Leakage × R)/D

where R is the DAC resistance at the VREF terminal.

For a DAC leakage current of 10 nA, R = 10 kΩ, and a gain

(that is, 1/D) of 16, the error voltage is 1.6 mV.

For D = 1 – 2–n the output voltage is

VOUT = –VIN/D = –VIN/(1 – 2–n)

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