Datasheet
AD5421 Data Sheet
Rev. G | Page 34 of 36
To determine the absolute worst-case overall error, the reference
and R
SET
errors can be directly summed with the specified AD5421
maximum error. For example, when using an external reference
and external R
SET
resistor, the maximum AD5421 error is 0.048%
of full-scale range. Assuming that the absolute errors for the
voltage reference and R
SET
resistor are, respectively, 0.04% and
0.05% with temperature coefficients of 3 ppm/°C and 2 ppm/°C,
respectively, the overall worst-case error is as follows:
Worst-Case Error =
AD5421 Error + V
REF
Absolute Error + V
REF
TC +
R
SET
Absolute Error + R
SET
TC
Worst-Case Error =
0.048% + 0.04% + [(3/10
6
) × 100 × 145]% +
0.05% + [(2/10
6
) × 100 × 145]% = 0.21% FSR
This is the absolute worst-case value when the AD5421 operates
over the temperature range of −40°C to +105°C. An error of this
value is very unlikely to occur because the temperature coeffi-
cients of the individual components do not exhibit the same
drift polarity, and, therefore, an element of cancelation occurs.
For this reason, the TC values should be added in a root of
squares fashion.
A further improvement can be gained by performing a two-point
calibration at zero scale and full scale, thus reducing the absolute
errors of the voltage reference and R
SET
resistor to a combined
error of 1 LSB or 0.0015% FSR. After performing this calibration,
the total maximum error becomes
Total Error =
FSR%102.0%)
029
.0
(%)
0435
.0
(%0015.0%048
.
0
22
=+
++
To reduce this error value further, a voltage reference and R
SET
resistor with lower TC specifications must be chosen.
THERMAL AND SUPPLY CONSIDERATIONS
The AD5421 is designed to operate at a maximum junction temp-
erature of 125°C. To ensure reliable and specified operation over
the lifetime of the product, it is important that the device not be
operated under conditions that cause the junction temperature
to exceed this value.
Excessive junction temperature can occur if the AD5421
experiences elevated voltages across its terminals while
regulating the loop current at a high value. The resulting
junction temperature depends on the ambient temperature.
Table 25 provides the bounds of operation at maximum ambient
temperature and maximum supply voltage. This information is
displayed graphically in Figure 52 and Figure 53. These figures
assume that the exposed paddle is connected to a copper plane
of approximately 6 cm
2
.
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0 20 40 60 80 100
POWER DISSIPATION (W)
AMBIENT TEMPERATURE (°C)
09128-103
TSSOP
LFCSP
Figure 52. Maximum Power Dissipation vs. Ambient Temperature
60
50
40
30
20
10
0
40 50 60 70
80 90
100
SUPPLY VOLTAGE (V)
AMBIENT TEMPERATURE (°C)
09128-102
LFCSP
TSSOP
Figure 53. Maximum Supply Voltage vs. Ambient Temperature
Table 25. Thermal and Supply Considerations (External MOSFET Not Connected)
Parameter Description 32-Lead LFCSP 28-Lead TSSOP
Maximum
Power
Dissipation
Maximum permitted power
dissipation when operating at an
ambient temperature of 105°C
mW500
40
105125
=
−
=
−
JA
AMAXJ
TT
θ
mW625
32
105125
=
−
=
−
JA
AJ MAX
θ
TT
Maximum
Ambient
Temperature
Maximum permitted ambient
temperature when operating from a
supply of 52 V while regulating a loop
current of 22.8 mA
=×−
JADMAXJ
P
T
θ
( )( )
C77400228.052125 °=××−
C87)32)0228.052((125
)(
°=××−
=×−
JAD MAXJ
PT
θ
Maximum
Supply
Voltage
Maximum permitted supply voltage
when operating at an ambient
temperature of 105°C while regulating
a loop current of 22.8 mA
V21
400228.0
105125
=
×
−
=
×
−
JALOOP
AMAXJ
I
TT
θ
V27
320228.0
105125
=
×
−
=
×
−
JALOOP
A MAXJ
I
TT
θ