EVAL-AD5445EBZ Analog Devices Inc, EVAL-AD5445EBZ Datasheet - Page 20

• Manufacturer Part Number: EVAL-AD5445EBZ
• Description: BOARD EVALUATION FOR AD5445
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD5445EBZ

• Number Of Dac's: 1
• Number Of Bits: 12
• Outputs And Type: 2, Single Ended
• Sampling Rate (per Second): 20M
• Data Interface: Parallel
• Settling Time: 30ns
• Dac Type: Current
• Voltage Supply Source: Single
• Operating Temperature: -40°C ~ 125°C
• Utilized Ic / Part: AD5445
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD5424/AD5433/AD5445
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
coefficients 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
OUT
Figure 54. Increasing the Gain of the Current Output DAC
V
REF
= –V
FB
resistor causes mismatches in the temperature
8-/10-/12-BIT
–n
IN
IN
the output voltage is
, gain can be added with an additional external
DAC
GND
V
V
/D = –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. B | Page 20 of 32
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
However, if 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 falls in the range
15.5 V
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.
Since only a fraction, D, of the current into the V
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.
IN
to 16.5 V
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
IN
—an error of 3% even though the DAC itself
1
2
Programmable Gain Element
R
FB
V
V
DD
DD
GND
V
REF
REF
terminal.
V
OUT
REF
terminal is
IN
.