Datasheet

Data Sheet AD5410/AD5420
Rev. F | Page 25 of 32
I
OUT
FILTERING CAPACITORS
Capacitors can be placed between CAP1 and AV
DD
, and CAP2
and AV
DD
, as shown in Figure 44.
AD5410/
AD5420
CAP1
CAP2
07027-037
AV
DD
AV
DD
GND
C1 C2
I
OUT
Figure 44. I
OUT
Filtering Capacitors
The capacitors form a filter on the current output circuitry, as
shown in Figure 45, reducing the bandwidth and the slew rate
of the output current. Figure 46 shows the effect the capacitors
have on the slew rate of the output current. To achieve significant
reductions in the rate of change, very large capacitor values are
required, which may not be suitable in some applications. In
this case, the digital slew rate control feature should be used.
The capacitors can be used in conjunction with the digital slew
rate control feature as a means of smoothing out the steps caused
by the digital code increments, as shown in Figure 47.
BOOST
CAP1
4kΩ
40Ω
DAC
12.5kΩ
R
SET
CAP2
I
OUT
C1
C2
AV
DD
07027-038
Figure 45. I
OUT
Filter Circuitry
0
5
10
15
20
25
–0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
OUTPUT CURRENT (mA)
TIME (ms)
NO CAPACITOR
10nF ON CAP1
10nF ON CAP2
47nF ON CAP1
47nF ON CAP2
07027-142
T
A
= 25°C
AV
DD
= 24V
R
LOAD
= 300Ω
Figure 46. Slew Controlled 4 mA to 20 mA Output Current Step Using
External Capacitors on the CAP1 and CAP2 Pins
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
–1 0
1 2
3
4 5
6 7
8
OUTPUT CURRENT (mA)
TIME (ms)
T
A
= 25°C
AV
DD
= 24V
R
LOAD
= 300Ω
NO EXTERNAL CAPS
10nF ON CAP1
10nF ON CAP2
07027-043
Figure 47. Smoothing Out the Steps Caused by the Digital Slew Rate Control
Feature
FEEDBACK/MONITORING OF OUTPUT CURRENT
For feedback or monitoring of the output current value, a sense
resistor can be placed in series with the I
OUT
output pin and the
voltage drop across it measured. As well as being an additional
component, the resistor increases the compliance voltage
required. An alternative method is to use a resistor that is
already in place. R3 is such a resistor and is internal to the
AD5410/AD5420, as shown in Figure 48. By measuring the
voltage between the R3
SENSE
and BOOST pins, the value of the
output current can be calculated as follows:
BIAS
R
OUT
I
R
V
I −=
3
3
(2)
where:
V
R3
is the voltage drop across R3 measured between the R3
SENSE
and BOOST pins.
I
BIAS
is a constant bias current flowing through R3 with a typical
value of 444 µA.
R3 is the resistance value of resistor R3 with a typical value of 40 Ω.
AV
DD
R
METAL
R3
40Ω
444µA
I
BIAS
R3
SENSE
I
OUT
BOOST
07027-050
Figure 48. Structure of Current Output Circuit