AD5424 Analog Devices, AD5424 Datasheet - Page 21

• Manufacturer Part Number: AD5424
• Description: High Bandwidth CMOS 8-Bit Parallel Interface Multiplying D/A Converter
• Manufacturer: Analog Devices

Specifications of AD5424

• Resolution (bits): 8bit
• Dac Update Rate: 20.4MSPS
• Dac Settling Time: 30ns
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

Data Sheet
ADDING GAIN
In applications where the output voltage is required to be
greater than V
amplifier or it can be achieved in a single stage. It is important
to consider the effect of the temperature coefficients of the thin
film resistors of the DAC. Simply placing a resistor in series with
the R
and results in larger gain temperature coefficient errors. Instead,
the circuit shown in Figure 54 is a recommended method of
increasing the gain of the circuit. R1, R2, and R3 should have
similar temperature coefficients, but they need not match
the temperature coefficients of the DAC. This approach is
recommended in circuits where gains greater than 1 are
required.
V
DACS 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 55, then the output voltage is
inversely proportional to the digital input fraction, D.
For D = 1 – 2
IN
NOTES:
1.
2.
ADDITIONAL PINS OMITTED FOR CLARITY
C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE
REQUIRED IF A1 IS A HIGH SPEED AMPLIFIER.
R1
V
FB
OUT
resistor causes mismatches in the temperature coefficients
Figure 54. Increasing the Gain of the Current Output DAC
V
= –V
REF
8-/10-/12-BIT
–n
IN
IN
the output voltage is
, gain can be added with an additional external
DAC
GND
/D = –V
V
V
DD
DD
IN
R
/(1 − 2
FB
I
I
OUT
OUT
1
2
–n
)
C1
FB
is used as the input
R3
R2
GAIN =
R1 =
V
R2 + R3
OUT
R2R3
R2 + R3
R2
Rev. C | Page 21 of 28
As D is reduced, the output voltage increases. For small values
of D, it is important to ensure that the amplifier does not saturate
and that the required accuracy is met.
For example, in the circuit shown in Figure 55, an 8-bit DAC
driven with the binary code 0x10 (00010000), that is, 16 decimal,
should cause the output voltage to be 16 × V
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 falls 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.
Since only a fraction, D, of the current into the V
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.
Output Error Voltage due to DAC Leakage = (Leakage × R)/D
IN
—an error of 3% even though the DAC itself has a
V
Figure 55. Current-Steering DAC Used as a Divider or
IN
OUT
I
I
OUT
OUT
NOTE:
ADDITIONAL PINS OMITTED FOR CLARITY
1 terminal, the output voltage has to change
1
2
Programmable Gain Element
R
FB
AD5424/AD5433/AD5445
V
V
DD
DD
GND
V
REF
REF
terminal.
IN
. However, if the
V
OUT
REF
terminal is
IN
to