AN780 MICROCHIP [Microchip Technology], AN780 Datasheet - Page 3

• Manufacturer Part Number: AN780
• Description: 15-Kilogram Scale Using the TC500A and the TC520
• Manufacturer: MICROCHIP [Microchip Technology]

FIGURE 3: Differential ratiometric reference voltage.
© 2002 Microchip Technology, Inc.
This application requires 120,000 counts which means that the
TC520's overrange bit must be used as the MSB, i.e., 17-bits. A
reference voltage with a tempco of 0.3ppm/ C would normally be
required for stability over a 30 C range. This could be a prohibitive
requirement. Fortunately the strain gauge has an output sensitivity
which is directly proportional to the supply voltage applied,
where K is the constant for a particular strain gauge and the dual
slope converter produces a result which is inversely proportional to
its reference voltage:
By deriving the reference from the supply voltage, any variations
will exactly cancel.
Strain Gauge
P
SG
–V
G = –R
V
IN
SG
= (V
V
T
V
DEINT
SG
REF
= K P
B
+
/R
= K (V
– V
= R
A
–
+
SG
= V
–
) P
REF
(V
IN
+
T
SG
+
– V
T
DEINT
x (V
– V
INT
K G
–
–
)P
/V
+
)
=
– V
SG
REF
V+
V–
V
V
,
V
REF
–
IN
SG
)/R
TOTAL
RA
T
INT
=
2
3
R
R
V
(V
REF
REF
TOTAL
–
+
7
4
(V
+
– V
= (V
+
= R
RB
– V
= R
–
) R
2B
–
6
+
) P
– V
1
REF/
+ R
+ R
SG
eq3/a
eq3/b
–
) R
3
R
2
K G
TOTAL
+ R
REF
3
/R
+ R
TOTAL
+
V
The TC500A has a differential reference input so the reference
voltage need not be referenced to ground. Rather than using a
precision reference for the TC500A and a precision supply for the
strain gauge, combining eq3/a and eq3/b into eq4 produces an
equation for the system:
Notice that V
pressure on the strain gauge (P
and the gain of the amplifier (G). The actual differential reference
voltage is determined only by the ratio of resistance values (R
R
IN
4
REF
T
INT
R2a
R2b
R3
R4
).
R1
= P
T
SG
DEINT
K G
IN
V
= K G P
REF
has been replaced by an expression for the
10
11
R
9
8
R
TOTAL
V
V
V
V
REF
C
IN+
IN–
REF+
REF–
SG
REF+
BUF
7
.68
T
4
R
INT
C
T
INT
REF–
INT
R
SG
TC500A
6
TOTAL
C
), the strain gage constant (K)
AZ
3
V–
C
C
INT
2
AZ
/R
Common
Analog
REF
C
1
INT
5
AN780
DS00780A-page 3
+5V
Ground
–5V
TOTAL
eq4
/