Datasheet

Data Sheet AD5424/AD5433/AD5445
Rev. D | Page 21 of 28
ADDING GAIN
In applications where the output voltage is required to be
greater than V
IN
, 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 R
FB
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.
03160-054
8-/10-/12-BIT
DAC
GND
V
DD
R
FB
V
DD
V
OUT
V
REF
V
IN
ADDITIONAL PINS OMITTED FOR CLARITY
C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE
REQUIRED IF A1 IS A HIGH SPEED AMPLIFIER.
NOTES:
1.
2.
C1
R1
I
OUT
1
I
OUT
2
R1 =
GAIN =
R2 + R3
R2
R3
R2
R2R3
R2 + R3
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 R
FB
is used as the input
resistor, as shown in Figure 55, then the output voltage is
inversely proportional to the digital input fraction, D.
For D = 1 – 2
–n
the output voltage is
V
OUT
= –V
IN
/D = –V
IN
/(1 − 2
–n
)
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.
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 × V
IN
. 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 V
IN
to
16.5 V
IN
—an error of 3% even though the DAC itself has a
maximum error of 0.2%.
03160-055
GND
R
FB
V
DD
V
DD
V
OUT
V
REF
V
IN
NOTE:
ADDITIONAL PINS OMITTED FOR CLARITY
I
OUT
1
I
OUT
2
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 V
REF
terminal is
routed to the I
OUT
1 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 V
REF
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.