Datasheet

Data Sheet AD5444/AD5446
Rev. E | Page 17 of 28
SINGLE-SUPPLY APPLICATIONS
Voltage Switching Mode of Operation
Figure 40 shows the AD5444/AD5446 DACs operating in the
voltage switching mode. The reference voltage (V
IN
) is applied
to the I
OUT
1 pin, I
OUT
2 is connected to AGND, and the output
voltage is available at the V
REF
terminal. In this configuration,
a positive reference voltage results in a positive output voltage,
making single-supply operation possible. The output from
the DAC is voltage at a constant impedance (the DAC ladder
resistance). Therefore, an op amp is necessary to buffer the
output voltage. The reference input no longer sees a constant
input impedance but rather one that varies with code, so the
voltage input should be driven from a low impedance source.
04588-032
NOTES
1. ADDITIONAL PINS OMITTED FOR CLARIT
Y.
2. C1 PHASE COMPENSATION (1pF
TO 2pF) MA
Y BE REQUIRED,
IF
A1 IS A HIGH SPEED AMPLIFIER.
I
OUT
1
GND
V
OUT
R2
V
IN
R
FB
V
DD
V
REF
R1
V
DD
Figure 40. Single-Supply Voltage Switching Mode Operation
It is important to note that, with this configuration, V
IN
is lim-
ited to low voltages, because the switches in the DAC ladder do
not have the same source-drain drive voltage. As a result, their
on resistance differs, which degrades the integral linearity of the
DAC. In addition, V
IN
must not go negative by more than 0.3 V,
or an internal diode turns on, exceeding the maximum ratings
of the device. In this type of application, the full range of the
multiplying capability of the DAC is lost.
Positive Output Voltage
The output voltage polarity is opposite to the V
REF
polarity for
dc reference voltages. To achieve a positive voltage output, an
applied negative reference to the input of the DAC is preferred
over the output inversion through an inverting amplifier because
of the resistor’s tolerance errors. To generate a negative reference,
the reference can be level-shifted by an op amp such that the
V
OUT
and GND pins of the reference become the virtual ground
and −2.5 V, respectively, as shown in Figure 41.
V
DD
R
FB
I
OUT
1
I
OUT
2
C1
V
OUT
= 0V T
O +2.5V
GND
V
DD
= +5V
V
REF
NOTES
1. ADDITIONAL PINS OMITTED FOR CLARITY.
2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED,
IFA1 IS
A HIGH SPEED AMPLIFIER.
ADR03
V
OUT
V
IN
GND
–5V
+5V
–2.5V
04588-033
Figure 41. Positive Voltage Output with Minimum Components
ADDING GAIN
In applications in which 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 take into consideration the effect of the temperature coeffi-
cients of the DAC’s thin film resistors. Simply placing a resistor
in series with the R
FB
resistor can cause mismatches in the
temperature coefficients and result in larger gain temperature
coefficient errors. Instead, increase the gain of the circuit by
using the recommended configuration shown in Figure 42.
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 where gains
of greater than 1 are required.
NOTES
1. ADDITIONAL PINS OMITTED FOR CLARIT
Y.
2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED,
IFA1 IS A
HIGH SPEED AMPLIFIER.
V
DD
R
FB
I
OUT
1
I
OUT
2
C1
V
OUT
GND
V
DD
V
REF
04588-034
GAIN =
R1 =
R2 + R3
R2
R2R3
R2 + R3
R1
V
IN
R3
R2
Figure 42. Increasing Gain of Current Output DAC
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 43, 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
)