Datasheet

Data Sheet ADE7854/ADE7858/ADE7868/ADE7878
Rev. G | Page 41 of 100
REFERENCE CIRCUIT
The nominal reference voltage at the REF
IN/OUT
pin is 1.2 V. This
is the reference voltage for the ADCs in the ADE7854/
ADE7858/ADE7868/ADE7878. Use a typical external reference
voltage of 1.2 V to overdrive the REF
IN/OUT
pin. The temperature
coefficient of the internal voltage reference is calculated based
on the endpoint method. To calculate the drift over
temperature, the values of the voltage reference at endpoints
(−40°C and +85°C) are measured and compared to the
reference value at 25°C, which in turn provides the slope of the
temperature coefficient curve. Figure 56 is a typical
representation of the drift over temperature. It contains two
curves: Curve X and Curve Y, which are typical representations
of two possible curvatures that are observed over the entire
specified temperature range.
+85°C
+25°C
–40°C
CURVE Y
CURVE X
+85°C–40°C
C'A'
REFERENCE VOLTAGE
TEMPERATURE (°C)
A
B
C
08510-555
Figure 56. Internal Voltage Reference Temperature Drift
Figure 56 shows that independent consideration of two regions
is necessary for accurate analysis of the drift over temperature,
as follows:
• Considering the region between Point A and Point B in
Curve X, the reference value increases with an increase in
temperature; thus, the curve has a positive slope from A to
B. This results in a positive temperature coefficient in this
region.
• Considering the region between Point B and Point C in
Curve X, the slope of the curve is negative because the
voltage reference decreases with an increase in
temperature; thus, this region of the curve has a negative
temperature coefficient.
• Based on similar logic, Curve Y has a negative temperature
coefficient between Point A’ and PointB and a positive
temperature coefficient between Point B and PointC’.
The drift curve on any particular IC can be matched with either
of these sample curves. The general relationship between the
absolute value of the voltage reference at a particular endpoint
temperature and the temperature coefficient for that region of
the curve is explained by the following two equations:
V
REF (−40°C)
= V
REF (+25°C)
×
( )






°−°−α
+
6
10
C25C40
1
c
V
REF (85°C)
= V
REF (25°C)
×
(
)






°−
°α
+
6
10
C25
C85
1
h
where α
c
and α
h
are cold and hot temperature coefficients,
respectively, calculated by
( )
C25C40
C)25(
C)25(C)40(
°−°−
−
=α
°+
°+°−
REF
REFREF
c
V
VV
× 10
6
ppm/°C
( )
C25C85
C)(25
C)(25C)85(
°−°
−
=α
°
°°
REF
REFREF
h
V
VV
× 10
6
ppm/°C
As the sign of cold and hot temperature coefficients can vary
from one IC to another, the typical drift is specified for the
whole range with a plus-minus sign(±). To find the typical,
minimum and maximum temperature coefficients, as listed in
the Specifications section, data based on the end-point method
is collected on ICs spread out over different lots. The minimum
and maximum temperature coefficents denote that the drift of
any particular IC will be within those limits, over the specified
temperature range, with reference to 25°C. See Figure 57 and
Figure 58 for the device-to-device variation of the drift.
08510-256
NUMBER OF PARTS
COLD TEMPERATURE COEFFICIENT (ppm/°C)
–50 –45 –40 –35 –30 –5 –20
–15 –10 –5 0 5 10 15 20 25 30 35 40 45 50
Figure 57. Histogram of the Reference Drift from −40°C to +25°C