AD5444 Analog Devices, AD5444 Datasheet - Page 17

• Manufacturer Part Number: AD5444
• Description: 12-Bit High Bandwidth Multiplying DAC with Serial Interface
• Manufacturer: Analog Devices

Specifications of AD5444

• Resolution (bits): 12bit
• Dac Update Rate: 2.7MSPS
• Dac Settling Time: 40ns
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Ser,SPI

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
to the I
voltage is available at the V
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.
It is important to note that, with this configuration, V
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
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
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
and −2.5 V, respectively, as shown in Figure 41.
OUT
V
IN
and GND pins of the reference become the virtual ground
OUT
Figure 40. Single-Supply Voltage Switching Mode Operation
NOTES
1. ADDITIONAL PINS OMITTED FOR CLARITY.
2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED,
IF A1 IS A HIGH SPEED AMPLIFIER.
1 pin, I
I
OUT
1
R
OUT
FB
IN
2 is connected to AGND, and the output
must not go negative by more than 0.3 V,
GND
V
V
DD
DD
REF
V
terminal. In this configuration,
REF
R1
R2
REF
IN
polarity for
) is applied
IN
V
is lim-
OUT
Rev. C | Page 17 of 28
ADDING GAIN
In applications in which the output voltage is required to be
greater than V
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
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.
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 43, then the output voltage is
inversely proportional to the digital input fraction, D .
For D = 1 − 2
V
IN
+5V
–5V
NOTES
1. ADDITIONAL PINS OMITTED FOR CLARITY.
2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED,
V
IF A1 IS A HIGH SPEED AMPLIFIER.
Figure 41. Positive Voltage Output with Minimum Components
OUT
R1
V
OUT
ADR03
GND
NOTES
1. ADDITIONAL PINS OMITTED FOR CLARITY.
2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED,
= − V
IF A1 IS A HIGH SPEED AMPLIFIER.
–2.5V
Figure 42. Increasing Gain of Current Output DAC
V
V
REF
−n
IN
IN
, the output voltage is
IN
, gain can be added with an additional external
/ D = − V
V
V
V
REF
GND
DD
DD
FB
V
resistor can cause mismatches in the
DD
V
GND
DD
= +5V
IN
R
/(1 − 2
FB
I
I
OUT
OUT
R
FB
1
2
−n
I
I
OUT
OUT
)
1
2
AD5444/AD5446
C1
FB
is used as the input
C1
R3
R2
V
GAIN =
R1 =
OUT
= 0V TO +2.5V
R2 + R3
V
R2R3
OUT
R2 + R3
R2