AD5441 Analog Devices, AD5441 Datasheet - Page 11

• Manufacturer Part Number: AD5441
• Description: 12-Bit Serial Input Multiplying DAC
• Manufacturer: Analog Devices

Specifications of AD5441

• Resolution (bits): 12bit
• Dac Update Rate: 5MSPS
• Dac Settling Time: 500ns
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Ser

V
PARAMETER DEFINITIONS
GENERAL CIRCUIT INFORMATION
The AD5441 is a 12-bit multiplying DAC with a low
temperature coefficient. It contains an R-2R resistor ladder
network, data input and control logic, and two data registers.
The digital circuitry forms an interface in which serial data can
be loaded under microprocessor control into a 12-bit shift register
and then transferred, in parallel, to the 12-bit DAC register.
The analog portion of the AD5441 contains an inverted R-2R
ladder network consisting of silicon-chrome, highly stable
(50 ppm/°C), thin-film resistors, and 12 pairs of NMOS current-
steering switches, see Figure 21. These switches steer binarily
weighted currents into either I
current in each ladder leg, regardless of digital input code. This
constant current results in a constant input resistance at V
equal to R. The V
or current, ac or dc, that is within the limits stated in the
Absolute Maximum Ratings.
The 12 output current steering NMOS FET switches are in
series with each R-2R resistor.
To further ensure accuracy across the full temperature range,
MOS switches that are always on were included in series with
the feedback resistor and the terminating resistor of the R-2R
ladder. Figure 21 shows the location of the series switches.
REF
*THESE SWITCHES PERMANENTLY ON.
NOTES
1. SWITCHES SHOWN FOR DIGITAL INPUTS HIGH.
BIT 1 (MSB)
20kΩ
S1
10kΩ
DIGITAL INPUTS
BIT 2
REF
20kΩ
S2
Figure 21. Simplified DAC Circuit
10kΩ
input may be driven by any reference voltage
BIT 3
20kΩ
S3
10kΩ
BIT 12 (LSB)
OUT
or GND; this yields a constant
20kΩ
S12
V
NOTES
1. R1 AND R2 USED ONLY IF GAIN ADJUSTMENT IS REQUIRED.
2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED
REF
IF A1 IS A HIGH SPEED AMPLIFIER.
R1
*
20kΩ
*
10kΩ
V
REF
LD
V
V
µCONTROLLER
Figure 22. Unipolar (2-Quadrant) Operation
DD
DD
GND
I
R
CLK
OUT
FEEDBACK
AD5441
REF
Rev. A | Page 11 of 16
SRI
R
FB
I
OUT
GND
R2
1
During any testing of the resistor ladder or R
incoming inspection), V
series switches.
OUTPUT IMPEDANCE
The output resistance of the AD5441, as in the case of the
output capacitance, varies with the digital input code. This
resistance, looking back into the I
10 kΩ (the feedback resistor alone when all digital inputs are
low) and 7.5 kΩ (the feedback resistor in parallel with approximate
30 kΩ of the R-2R ladder network resistance when any single bit
logic is high). Static accuracy and dynamic performance are
affected by these variations.
APPLICATIONS INFORMATION
In most applications, linearity depends upon the potential of
the I
DAC is connected to an external precision op amp inverting input.
The external amplifiers noninverting input should be tied directly
to ground without the usual bias current compensating resistor (see
Figure 22 and Figure 24). The selected amplifier should have a low
input bias current and low drift over temperature. The amplifiers
input offset voltage should be nulled to less than 200 mV (less than
10% of 1 LSB). All grounded pins should tie to a single common
ground point to avoid ground loops. The V
have a low noise level with adequate bypassing. It is best to operate
the AD5441 from the analog power supply and grounds.
UNIPOLAR 2-QUADRANT MULTIPLYING
The most straightforward application of the AD5441 is in the
2-quadrant multiplying configuration shown in Figure 22. If the
reference input signal is replaced with a fixed dc voltage reference,
the DAC output provides a proportional dc voltage output
according to the transfer equation
where:
D is the decimal data loaded into the DAC register.
V
REF
AGND
OUT
is the externally applied reference voltage source.
V
C1
OUT
and GND pins being at the same voltage potential. The
A1
= −D/4096 × V
V
OUT
= 0 TO –V
REF
DD
REF
must be present to turn on these
OUT
terminal, may be between
DD
power supply should
FEEDBACK
AD5441
(such as