EVAL-AD5449EBZ datasheet(18/32 Pages) AD

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AD5429/AD5439/AD5449

Rev. C | Page 18 of 32

ADDING GAIN

In applications in which the output voltage must be greater than

VIN, gain can be added with an additional external amplifier, or

it can be achieved in a single stage. Consider the effect of temper-

ature 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 temper-

ature coefficient errors. Instead, the circuit in Figure 42 shows

the 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 in which

gains of greater than 1 are required.

DIVIDER OR PROGRAMMABLE GAIN ELEMENT

Current-steering DACs are very flexible and lend themselves

to many applications. If this type of DAC is connected as the

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

resistor, as shown in Figure 43, the output voltage is inversely

proportional to the digital input fraction, D.

For D = 1 − 2−n, 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 the required accuracy is met. For example,

an 8-bit DAC driven with binary code of 0x10 (0001 0000)—that

is, 16 decimal—in the circuit of Figure 43 should cause the output

voltage to be 16 × VIN. However, if the DAC has a linearity speci-

fication of ±0.5 LSB, D can have a weight in the range of 15.5/256

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

15.5 VIN to 16.5 VIN. This range represents 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 VREFx terminal

is routed to the IOUT1 terminal, the output voltage changes as

follows:

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

where R is the DAC resistance at the VREFx 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

RFBA

IOUT1A

IOUT2A

GND

VDD

VREFA

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 42. Increasing Gain of Current Output DAC

VIN

NOTES

1. ADDITIONAL PINS OMITTED FOR CLARITY.

VREFA

VDD

RFBA

IOUT1A

IOUT2A

GND

VOUT

8-/10-/12-BIT

DAC

Figure 43. Current-Steering DAC Used as a Divider or Programmable Gain Element

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