AD8021 datasheet(18/32 Pages) AD

18 / 32 page

AD5429/AD5439/AD5449

Rev. 0 | Page 18 of 32

ADDING GAIN

In applications in which 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. Be sure to take

into consideration the effect of 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, resulting in larger gain temperature coefficient

errors. Instead, the circuit of Figure 41 is a recommended

method of increasing the gain of the circuit. R1, R2, and R3

should all have similar temperature coefficients, but they need

not match the temperature coefficients of the DAC. This

approach is recommended in circuits in which gains of > 1

are required.

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 42, then the output voltage is

inversely proportional to the digital input fraction D. For

D = 1 − 2n the output voltage is

()n

OUT

−

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

of the digital fraction D, it is important to ensure that the

amplifier does not saturate and also that the required accuracy

is met. For example, an 8-bit DAC driven with the binary code

0 × 10 (00010000)—that is, 16 decimal—in the circuit of

Figure 42 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 in the range 15.5/256 to

16.5/256, so that the possible output voltage is in the range

15.5 VIN to 16.5 VIN with an error of +3%, even though the DAC

itself has a maximum error of 0.2%.

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.

Because 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:

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.

VDD

RFB

IOUT1

IOUT2

GND

VDD

VREF

NOTES:

1. ADDITIONAL PINS OMITTED FOR CLARITY.

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

8-/10-/12-BIT

DAC

VIN

VOUT

R1 =

R2R3

R2 + R3

GAIN =

R2 + R3

IF A1 IS A HIGH SPEED AMPLIFIER.

Figure 41. Increasing Gain of Current Output DAC

VIN

NOTE:

1. ADDITIONAL PINS OMITTED FOR CLARITY.

VREF

VDD

RFB

IOUT1

IOUT2

GND

VOUT

Figure 42. Current-Steering DAC Used as a Divider

or Programmable Gain Element

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