AD5415YRU-REEL AD [Analog Devices], AD5415YRU-REEL Datasheet - Page 18

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

AD5415
As D is reduced, the output voltage increases. For small values
of the digital fraction, D, it is important to ensure that the
amplifier does not saturate and also that the required accuracy
is met. For example, an 8-bit DAC driven with the binary code
0x10 (0001 0000), that is, 16 decimal, in the circuit of Figure 37
should cause the output voltage to be 16 times V
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 is in the range 15.5 V
16.5 V
maximum error of 0.2%.
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.
Because only a fraction D of the current into the V
is routed to the I
as follows:
where R is the DAC resistance at the V
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.
REFERENCE SELECTION
When selecting a reference for use with the AD54xx series of
current output DACs, pay attention to the reference’s output
voltage temperature coefficient specification. This parameter
affects not only the full-scale error, but can also affect the
linearity (INL and DNL) performance. The reference tempera-
ture coefficient should be consistent with the system accuracy
specifications. For example, an 8-bit system required to hold its
Output Error Voltage Due to DAC Leakage
IN
, an error of 3% even though the DAC itself has a
Figure 38. Current-Steering DAC Used as a Divider or
V
IN
NOTE:
1
ADDITIONAL PINS OMITTED FOR CLARITY.
OUT
I
I
OUT
OUT
1 terminal, the output voltage has to change
Programmable Gain Element
2
1
R
FB
GND
V
V
DD
DD
V
REF
REF
terminal.
= (
V
Leakage
IN
OUT
. However, if
REF
terminal
IN
×
to
R
)/
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D
overall specification to within 1 LSB over the temperature range
0°C to 50°C dictates that the maximum system drift with
temperature should be less than 78 ppm/°C. A 12-bit system
with the same temperature range to overall specification within
2 LSB requires a maximum drift of 10 ppm/°C. By choosing a
precision reference with a low output temperature coefficient,
this error source can be minimized. Table 7 suggests some of
the references available from Analog Devices that are suitable
for use with this range of current output DACs.
AMPLIFIER SELECTION
The primary requirement for the current-steering mode is an
amplifier with low input bias currents and low input offset
voltage. The input offset voltage of an op amp is multiplied by
the variable gain (due to the code-dependent output resistance
of the DAC) of the circuit. A change in this noise gain between
two adjacent digital fractions produces a step change in the
output voltage due to the amplifier’s input offset voltage. This
output voltage change is superimposed upon the desired change
in output between the two codes and gives rise to a differential
linearity error, which, if large enough, could cause the DAC to
be nonmonotonic.
The input bias current of an op amp also generates an offset at
the voltage output as a result of the bias current flowing in the
feedback resistor, R
low enough to prevent any significant errors in 12-bit
applications.
Common-mode rejection of the op amp is important in voltage
switching circuits, because it produces a code-dependent error
at the voltage output of the circuit. Most op amps have adequate
common-mode rejection for use at 12-bit resolution.
Provided that the DAC switches are driven from true wideband
low impedance sources (V
Consequently, the slew rate and settling time of a voltage
switching DAC circuit is determined largely by the output op
amp. To obtain minimum settling time in this configuration, it
is important to minimize capacitance at the V
output node in this application) of the DAC. This is done by
using low inputs, capacitance buffer amplifiers, and careful
board design.
Most single-supply circuits include ground as part of the analog
signal range, which in turn requires an amplifier that can handle
rail-to-rail signals. A large range of single-supply amplifiers is
available from Analog Devices.
FB
. Most op amps have input bias currents
IN
and AGND), they settle quickly.
REF
node (voltage