AD5405YCP-REEL7 Analog Devices, AD5405YCP-REEL7 Datasheet - Page 16

• Manufacturer Part Number: AD5405YCP-REEL7
• Description: Dual 12-Bit/ High Bandwidth/ Multiplying DAC with 4-Quadrant Resistors and Parallel Interface
• Manufacturer: Analog Devices

AD5405
USED AS A DIVIDER OR PROGRAMMABLE GAIN
ELEMENT
Used as a 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 37, then the output voltage is
inversely proportional to the digital input fraction D.
For D = 1 − 2
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
0 × 10 (00010000), that is, 16 decimal, in the circuit of Figure 37
should cause the output voltage to be 16 × V
DAC has a linearity specification of ±0.5 LSB, then D can, in
fact, have the 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:
Output Error Voltage Due to DAC Leakage = ( Leakage × R)/D
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.
V
OUT
IN
—an error of 3% even though the DAC itself has a
=
NOTE
ADDITIONAL PINS OMITTED FOR CLARITY
Figure 37. Current-Steering DAC Used as a Divider or
−
V
n
V
IN
the output voltage is
IN
OUT
D
I
I
OUT
OUT
=
1 terminal, the output voltage has to change
Programmable Gain Element
−
1
2
R
V
FB
IN
V
GND
V
(
DD
DD
1
−
2
−
n
)
V
REF
FB
REF
is used as the input
terminal.
IN
. However, if the
V
OUT
REF
terminal
IN
to
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REFERENCE SELECTION
When selecting a reference for use with the AD5405 series of
current output DACs, pay attention to the reference output
voltage temperature coefficient specification. This parameter
not only affects the full-scale error, but can also affect the
linearity (INL and DNL) performance. The reference temper-
ature coefficient should be consistent with the system accuracy
specifications. For example, an 8-bit system required to hold
its 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 LSBs requires a maximum drift of 10 ppm/°C. By choosing a
precision reference with low output temperature coefficient, this
error source can be minimized. Table 7 lists some 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
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 the DAC switches are driven from true wide band,
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,
minimize capacitance at the V
this application) of the DAC. This is done by using low input
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. Analog Devices offers a large range of single-
supply amplifiers, as listed in Table 8.
FB
. Most op amps have input bias currents low
IN
and AGND) they settle quickly.
REF
node (voltage output node in