Datasheet

AD5429/AD5439/AD5449 Data Sheet
Rev. E | Page 18 of 28
ADDING GAIN
In applications in which the output voltage must be greater than
V
IN
, 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 R
FB
A 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
ININ
OUT
V
DVV
/
/
/=/= 21/
/
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 × V
IN
. 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 V
IN
to 16.5 V
IN
. 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 V
REF
x terminal
is routed to the I
OUT
1 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 V
REF
x 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.
V
DD
R
FB
A
I
OUT
1A
I
OUT
2A
C1
GND
V
DD
V
REF
A
NOTES
1. ADDITIONAL PINS OMITTED FOR CLARITY.
2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED
8-/10-/12-BIT
DAC
V
IN
R1
R3
R2
V
OUT
R1 =
R2R3
R2 + R3
GAIN =
R2 + R3
R2
04464-011
IF A1 IS A HIGH SPEED AMPLIFIER.
Figure 42. Increasing Gain of Current Output DAC
V
IN
NOTES
1. ADDITIONAL PINS OMITTED FOR CLARITY.
V
REF
A
V
DD
V
DD
R
FB
A
I
OUT
1A
I
OUT
2A
GND
V
OUT
04464-012
8-/10-/12-BIT
DAC
Figure 43. Current-Steering DAC Used as a Divider or Programmable Gain Element