Datasheet

© 2008 Microchip Technology Inc. DS21428E-page 19
TC500/A/510/514
9.0 DESIGN EXAMPLES
Refer to Figures 9-1 to 9-4.
EQUATION 9-1:
Given: Required Resolution: 16 bits (65,536
counts).
Maximum V
IN
: ±2V
Power Supply Voltage: +5V
60 Hz System
Step 1. Pick integration time (t
INT
) as a multiple
of the line frequency:
1/60 Hz = 16.6 ms. Use 4x line
frequency.
= 66 ms
Step 2. Calculate R
INT
:
R
INT
= V
IN(MAX)
/20 μA 2 /20 μA
= 100 kΩ
Step 3. Calculate C
INT
for maximum (4V)
integrator output swing.
C
INT
= (t
INT
) (20 x 10
–6
) / (V
S
- 0.9)
= (.066) (20 x 10
–6
) / (4.1)
= 0.32 μF (use closest value: 0.33 μF)
Note: Microchip recommended capacitor:
Evox Rifa p/n: 5MR5 334K50J03L4.
Step 4. Choose C
REF
and C
AZ
based on
conversion rate.
Conversions/sec:
= 1/(T
AZ
+ T
INT
+ 2 T
INT
+ 2 ms)
= 1/(66 ms +66 ms +132 ms +2 ms)
= 3.7 conversions/sec
From which C
AZ
= C
REF
= 0.22 μF
(see Table 7-1)
Note: Microchip recommended capacitor:
Evox Rifa p/n: 5MR5 224K50J02L4
Step 5. Calculate V
REF
:
V
REF
V
S
0.9–()C
INT
()R
INT
()
2T
INT
()
-----------------------------------------------------------
=
4.1()0.33 10
6–
× ()100 10
3
× ()
2 66 10
3–
× ()
--------------------------------------------------------------------------=
1.025 V()=