EVAL-AD5429EBZ Analog Devices Inc, EVAL-AD5429EBZ Datasheet - Page 18

• Manufacturer Part Number: EVAL-AD5429EBZ
• Description: BOARD EVALUATION FOR AD5429
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD5429EBZ

• Number Of Dac's: 2
• Number Of Bits: 8
• Outputs And Type: 2, Single Ended
• Sampling Rate (per Second): 2.47M
• Data Interface: Serial
• Settling Time: 30ns
• Dac Type: Current
• Voltage Supply Source: Single
• Operating Temperature: -40°C ~ 125°C
• Utilized Ic / Part: AD5429
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD5429/AD5439/AD5449
ADDING GAIN
In applications in which the output voltage must be greater than
V
it can be achieved in a single stage. Consider the effect of temper-
ature coefficients of the thin film resistors of the DAC. Simply
placing a resistor in series with the RFB resistor causes mismatches
in the temperature coefficients, resulting in larger gain temper-
ature coefficient errors. Instead, the circuit in Figure 42 shows
the 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 in which
gains of greater than 1 are required.
DIVIDER OR PROGRAMMABLE GAIN ELEMENT
Current-steering DACs are very flexible and lend themselves
to many applications. If this type of DAC is connected as the
feedback element of an op amp and R
resistor, as shown in Figure 43, the output voltage is inversely
proportional to the digital input fraction, D.
For D = 1 − 2
IN
, gain can be added with an additional external amplifier, or
V
OUT
=
−
−n
V
, the output voltage is
IN
/
D
=
−
V
IN
/
(
1
−
2
Figure 43. Current-Steering DAC Used as a Divider or Programmable Gain Element
−
n
FB
V
)
IN
A is used as the input
R1
NOTES
1. ADDITIONAL PINS OMITTED FOR CLARITY.
2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED
IF A1 IS A HIGH SPEED AMPLIFIER.
V
Figure 42. Increasing Gain of Current Output DAC
V
NOTES
1. ADDITIONAL PINS OMITTED FOR CLARITY.
IN
REF
A
8-/10-/12-BIT
V
V
GND
DD
DD
I
I
OUT
OUT
DAC
2A
1A
R
Rev. C | Page 18 of 32
FB
8-/10-/12-BIT
R
A
FB
I
I
DAC
A
OUT
OUT
GND
1A
2A
V
V
DD
DD
V
REF
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 the required accuracy is met. For example,
an 8-bit DAC driven with binary code of 0x10 (0001 0000)—that
is, 16 decimal—in the circuit of Figure 43 should cause the output
voltage to be 16 × V
fication of ±0.5 LSB, D can have a weight in the range of 15.5/256
to 16.5/256, so that the possible output voltage is in the range of
15.5 V
though the DAC itself has a 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
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.
A
C1
Output Error Voltage Due to DAC Leakage = (Leakage × R)/D
IN
to 16.5 V
R3
R2
V
OUT
GAIN =
R1 =
V
OUT
R2 + R3
IN
OUT
R2R3
. This range represents an error of 3%, even
R2 + R3
1 terminal, the output voltage changes as
IN
R2
. However, if the DAC has a linearity speci-
REF
x terminal.
REF
x terminal