EVAL-AD5445EB Analog Devices Inc, EVAL-AD5445EB Datasheet - Page 19

• Manufacturer Part Number: EVAL-AD5445EB
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD5445EB

• Lead Free Status / Rohs Status: Not Compliant

SINGLE-SUPPLY APPLICATIONS
Current Mode Operation
The current mode circuit in Figure 51 shows a typical circuit for
operation with a single 2.5 V to 5 V supply. I
I
configuration, the output voltage is given by
As D varies from 0 to 255 (AD5424), 0 to 1023 (AD5433),
or 0 to 4095 (AD5445), the output voltage varies from
V
sourcing all possible variations in current at the I
It is important to note that V
the switches in the DAC ladder no longer have the same source-
drain drive voltage. As a result, their on resistance differs and
the linearity of the DAC degrades.
Voltage Switching Mode of Operation
Figure 52 shows these DACs operating in the voltage-switching
mode. The reference voltage, V
I
at the V
voltage results in a positive output voltage, making single-
supply operation possible. The output from the DAC is voltage
at a constant impedance (the DAC ladder resistance), thus an op
amp is necessary to buffer the output voltage. The reference
input no longer sees a constant input impedance, but one that
varies with code. So, the voltage input should be driven from a
low impedance source.
OUT
OUT
V
BIAS
IN
1 is biased positive by the amount applied to V
2 is connected to AGND, and the output voltage is available
V
V
should be a low impedance source capable of sinking and
OUT
OUT
REF
NOTES:
1.
2.
= [ D × ( R
= V
ADDITIONAL PINS OMITTED FOR CLARITY
C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED
IF A1 IS A HIGH SPEED AMPLIFIER.
terminal. In this configuration, a positive reference
Figure 51. Single-Supply Current Mode Operation
BIAS
V
REF
to V
FB
V
GND
V
/R
OUT
DD
DD
DAC
V
BIAS
DAC
= 2V
) × ( V
IN
BIAS
R
IN
is limited to low voltages because
FB
I
I
OUT
OUT
, is applied to the I
BIAS
− V
1
2
– V
IN
.
IN
)] + V
C1
OUT
A1
2 and therefore
BIAS
OUT
BIAS
OUT
2 terminal.
. In this
1 pin,
V
OUT
Rev. A | Page 19 of 32
It is important to note that V
the switches in the DAC ladder no longer have the same source-
drain drive voltage. As a result, their on resistance differs, which
degrades the linearity of the DAC. See Figure 18 to Figure 23.
Also, V
internal diode turns on, exceeding the maximum ratings of the
device. In this type of application, the full range of multiplying
capability of the DAC is lost.
POSITIVE OUTPUT VOLTAGE
Note that the output voltage polarity is opposite to the V
polarity for dc reference voltages. To achieve a positive voltage
output, an applied negative reference to the input of the DAC is
preferred over the output inversion through an inverting ampli-
fier because of the resistor tolerance errors. To generate a
negative reference, the reference can be level-shifted by an op
amp such that the V
the virtual ground and –2.5 V respectively, as shown
in Figure 53.
V
IN
+5V
–5V
Figure 53. Positive Voltage Output with Minimum of Components
V
Figure 52. Single-Supply Voltage-Switching Mode Operation
IN
OUT
ADR03
GND
NOTES:
1
2
must not go negative by more than 0.3 V,;otherwise, an
ADDITIONAL PINS OMITTED FOR CLARITY.
C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED,
IF A1 IS A HIGH SPEED AMPLIFIER.
NOTES:
–2.5V
1.
2.
I
I
OUT
OUT
V
ADDITIONAL PINS OMITTED FOR CLARITY
C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED
IF A1 IS A HIGH SPEED AMPLIFIER.
IN
1
2
R
V
FB
REF
OUT
V
DAC
DD
V
V
GND
V
GND
DD
DD
and GND pins of the reference become
DD
= 5V
AD5424/AD5433/AD5445
IN
V
R
is limited to low voltages because
REF
FB
I
I
OUT
OUT
1
2
R1
C1
A1
R2
V
OUT
= 0V TO +2.5V
REF
V
OUT