TC820 TelCom Semiconductor, TC820 Datasheet - Page 18

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

nominal 2V integrator swing. An exact expression for C
is:
where: f
roll-over error. A polypropylene capacitor is recommended.
Integrating Resistor — R
with class A output stages. The integrator and buffer can
supply 40 A drive currents with negligible linearity errors.
R
region but not so large that printed circuit board leakage
currents induce errors. For a 400mV full scale, R
be about 100k .
3-166
TC820
INT
COMPONENTS
COMPONENTS
C
C
The input buffer amplifier and integrator are designed
OSCILLATOR
OSCILLATOR
is chosen to remain in the output stage linear drive
INT
INT
TC820
V
R
V
OSC
=
must have low dielectric absorption to minimize
FS
INT
INT
XTAL
V
R/C
= Clock frequency
= Full-scale input voltage
= Integrating resistor
= Desired full-scale integrator output swing
Figure 17. System Clock Generation
INT
4000 V
R
OSC1
INT
FS
f
BACKPLANE
OSC
TIME BASE
COUNTER
COUNTER
INT
BUZZER
DRIVER
LCD
OSC2
A/D
OSC3
40,000
240
2
8
INT
should
INT
exist between a transducer output voltage and the required
digital reading. Assume, for example, that a pressure trans-
ducer output is 800mV for 4000 lb/in
the input voltage by two, the reference voltage should be set
to 400mV. This permits the transducer input to be used
directly.
analog common will normally be used to supply the
converter's reference voltage. This potential is stable when-
ever the supply potential is greater than approximately 7V.
The low-battery detection circuit and analog common oper-
ate from the same internal reference. This ensures that the
low-battery annunciator will turn on at the time the internal
reference begins to lose regulation.
Figure 18 shows internal and external reference applica-
tions.
Ratiometric Resistance Measurements
ence make ratiometric readings possible. In ratiometric
operation, an unknown resistance is measured with respect
to a known standard resistance. No accurately defined
reference voltage is needed.
standard and a current is passed through the pair (Fig-
ure 19). The voltage developed across the unknown is
applied to the input and voltages across the known resistor
Reference Voltage Selection
the input signal be twice the reference voltage.
Table III. Reference Voltage Selection
NOTES: 1. TC820 in A/D converter mode, RANGE/FREQ = logic low.
Input Voltage
(V
(Notes 3, 4)
Full-Scale
FS
In some applications, a scale factor other than unity may
The internal voltage reference potential available at
The TC820 can also operate with an external reference.
The TC820 true differential input and differential refer-
The unknown resistance is put in series with a known
A full-scale reading (4000 counts for TC820) requires
200mV
400mV
) (Note 1)
1V
2V
2. Not recommended.
3. V
4. Full-scale voltage values are not limited to the values
See "10:1 Range Change" section.
shown. For example, TC820 V
400mV to 2V.
FS
> 2V may exceed the input common mode range.
3-3/4 A/D CONVERTER WITH
TELCOM SEMICONDUCTOR, INC.
FREQUENCY COUNTER
200mV
500mV
Note 2
V
1V
REF
AND LOGIC PROBE
FS
2
. Rather than dividing
can be any value from
Resolution
250 V
500 V
10 V
–