TC820 TelCom Semiconductor, TC820 Datasheet - Page 17

• Manufacturer Part Number: TC820
• Description: 3-3/4 DIGIT A/D CONVERTER WITH FREQUENCY COUNTER AND LOGIC PROBE
• Manufacturer: TelCom Semiconductor

3-3/4 A/D CONVERTER WITH
FREQUENCY COUNTER
AND LOGIC PROBE
Clock Oscillator
crystal or with an inexpensive resistor-capacitor combina-
tion. The crystal circuit, shown in Figure 15, is recommended
when high accuracy is required in the frequency counter
mode. The 40kHz crystal is a standard frequency for ultra-
sonic alarms, and will provide a 1-second time base for the
counter or 2.5 analog-to-digital conversions per second.
Consult the crystal manufacturer for detailed applications
information.
can be used. The frequency of this circuit will be approxi-
mately:
The TC820 oscillator can be controlled with either a
Where low cost is important, the R-C circuit of Figure 16
f
OSC
TELCOM SEMICONDUCTOR, INC.
=
Figure 15. Suggested Crystal Oscillator Circuit
TC820
–
+
37
RC
12 A
0.3
Figure 14. DGND and COM Outputs
5 pF
22 M
40kHz
N
N
10pF
SECTION
38
LOGIC
470k
P
5V
TC820
3.2V
39
V
V
COM
DGND
DD
SS
value should be 100k . For accurate frequency measure-
ment, an R-C oscillator frequency of 40kHz is required.
System Timing
clock oscillator is divided by 2 prior to clocking the A/D
counters. The clock is also divided by 8 to drive the buzzer,
by 240 to generate the LCD backplane frequency, and by
40,000 for the frequency counter time base. A simplified
diagram of the system clock is shown in Figure 17.
Component Value Selection
Auto Zero Capacitor — C
influence on system noise. A 0.47 F capacitor is recom-
mended; a low dielectric absorption capacitor (Mylar) is
required.
Reference Voltage Capacitor — C
grator output voltage back to zero during the reference
integrate cycle is stored on C
typical. A good quality, low leakage capacitor (such as
Mylar) should be used.
Integrating Capacitor — C
voltage swing without causing output saturation. Analog
common will normally supply the differential voltage
reference. For this case, a 2V integrator output swing is
optimum when the analog input is near full scale. For 2.5
readings/second (f
value is suggested. If a different oscillator frequency is used,
C
INT
Typical values are R = 10k and C = 68pF. The resistor
All system timing is derived from the clock oscillator. The
The value of the auto-zero capacitor (C
The reference voltage capacitor used to ramp the inte-
C
must be changed in inverse proportion to maintain the
INT
should be selected to maximize integrator output
37
5pF
Figure 16. R-C Oscillator Circuit
OSC
= 40kHz) and V
110k
AZ
INT
10pF
REF
38
. A 0.1 F capacitor is
REF
75pF
FS
= 400mV, a 0.22 F
TC820
AZ
39
) has some
TC820
3-165
1
2
3
4
5
6
7
8