AD5415YRU-REEL AD [Analog Devices], AD5415YRU-REEL Datasheet - Page 17

• Manufacturer Part Number: AD5415YRU-REEL
• Description: Dual 12-Bit, High Bandwidth, Multiplying DAC with 4-Quadrant Resistors and Serial Interface
• Manufacturer: AD [Analog Devices]

SINGLE-SUPPLY APPLICATIONS
VOLTAGE SWITCHING MODE OF OPERATION
Figure 35 shows these DACs operating in the voltage switching
mode. The reference voltage, V
I
at the V
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
one that varies with code. So, the voltage input should be driven
from a low impedance source.
Note that V
the DAC ladder no longer have the same source-drain drive
voltage. As a result, their on resistance differs and this degrades
the integral linearity of the DAC. Also, V
by more than 0.3 V or an internal diode is turned on, exceeding
the maximum ratings of the device. In this type of application,
the full range of multiplying capability of the DAC is lost.
POSITIVE OUTPUT VOLTAGE
The output voltage polarity is opposite to the V
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 resistors’ tolerance errors. To generate a negative
reference, the reference can be level-shifted by an op amp such
that the V
ground and −2.5 V, respectively, as shown in Figure 36.
OUT
V
NOTES
1. SIMILAR CONFIGURATION FOR DACB
2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED
IN
IF A1 IS A HIGH SPEED AMPLIFIER.
2 is connected to AGND, and the output voltage is available
REF
OUT
terminal. In this configuration, a positive reference
Figure 35. Single-Supply Voltage Switching Mode
IN
I
I
OUT
OUT
and GND pins of the reference become the virtual
is limited to low voltages, because the switches in
1
2
R
FB
GND
V
V
DD
DD
V
REF
IN
, is applied to the I
R
1
IN
must not go negative
R
REF
2
polarity for
OUT
1 pin,
V
OUT
Rev. 0 | Page 17 of 28
ADDING GAIN
In applications where the output voltage is required to be
greater than V
amplifier, or it can also be achieved in a single stage. It is
important 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 R
temperature coefficients, resulting in larger gain temperature
coefficient errors. Instead, the circuit in Figure 37 is a recom-
mended method of increasing the gain of the circuit. R
R
need not match the temperature coefficients of the DAC. This
approach is recommended in circuits where gains of greater
than 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 R
resistor, as shown in Figure 38, then the output voltage is
inversely proportional to the digital input fraction, D. For D
equal to 1 − 2
V
3
IN
should all have similar temperature coefficients, but they
+5V
–5V
Figure 36. Positive Voltage Output with Minimum of Components
V
1/2 AD8552
R2
OUT
V
NOTES:
1
2
OUT
ADR03
ADDITIONAL PINS OMITTED FOR CLARITY.
C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED,
Figure 37. Increasing the Gain of the Current Output DAC
IF A1 IS A HIGH SPEED AMPLIFIER.
NOTES:
1
2
GND
ADDITIONAL PINS OMITTED FOR CLARITY.
C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED,
IF A1 IS A HIGH SPEED AMPLIFIER.
= −V
–2.5V
V
V
IN
n
REF
IN
, the output voltage is
IN
, gain can be added with an additional external
/D = −V
V
REF
V
V
12-BIT DAC
GND
DD
DD
V
DD
12-BIT DAC
V
GND
DD
= 5V
IN
/(1 −2
R
FB
FB
I
I
OUT
OUT
R
resistor causes mismatches in the
FB
−n
1
2
I
I
OUT
OUT
)
1
2
C1
FB
C1
is used as the input
1/2 AD8552
R3
R2
V
GAIN =
R1 =
OUT
AD5415
= 0 TO +2.5V
V
R2 + R3
1
OUT
, R
R2R3
R2 + R3
2
R2
, and