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TC820

3-3/4 DIGIT A/D CONVERTER WITH FREQUENCY COUNTER AND LOGIC PROBE

TelCom Semiconductor Inc => Microchip 

3-3/4 A/D CONVERTER WITH

FREQUENCY COUNTER

AND LOGIC PROBE

TC820

Reference Integrate (Deintegrate) Phase

The reference capacitor, which was charged during the

auto-zero phase, is connected to the input of the integrating

amplifier. The internal sign logic ensures the polarity of the

reference voltage is always connected in the phase opposite

to that of the input voltage. This causes the integrator to

ramp back to zero at a constant rate determined by the

reference potential.

The amount of time required (TDEINT) for the integrating

amplifier to reach zero is directly proportional to the ampli-

tude of the voltage that was put on the integrating capacitor

(VINT) during the integration phase:

t DEINT = RINT CINT VINT

VREF

The digital reading displayed by the TC820 is:

Digital Count = 2000 VI+N – VI–N

VREF

System Timing

The oscillator frequency is divided by 2 prior to clocking

the internal decade counters. The four-phase measurement

cycle takes a total of 8000 (4000) counts or 16000 clock

pulses. The 8000 count phase is independent of input signal

magnitude or polarity.

Each phase of the measurement cycle has the following

length:

Conversion Phase

Counts

1) Auto-Zero:

2) Signal Integrate:1,2

3) Reference Integrate:

4) Integrator Output Zero:

1500

2000

1 to 4001

499 to 4499

NOTES: 1. This time period is fixed. The integration period for the

TC820 is:

tINT (TC820) = 4000 = 2000 counts

fOSC

where fOSC is the clock oscillator frequency.

2. Times shown are the RANGE/FREQ at logic low (normal

operation). When RANGE/FREQ is logic high, signal

integrate times are 200 counts. See "10:1 Range Change"

section.

Input Overrange

When the analog input is greater than full scale, the LCD

will display "OL" and the "OVERRANGE" LCD annunciator

will be on.

3-158

Peak Reading Hold

The TC820 provides the capability of holding the highest

(or peak) reading. Connecting the PK HOLD input to VDD

enables the peak hold feature. At the end of each conversion

the contents of the TC820 counter is compared to the

contents of the display register. If the new reading is higher

than the reading being displayed, the higher reading is

transferred to the display register. A "higher" reading is

defined as the reading with the higher absolute value.

The peak reading is held in the display register so the

reading will not "droop" or slowly decay with time. The held

reading will be retained until a higher reading occurs, the PK

HOLD input is disconnected from VDD, or power is removed.

The peak signal to be measured must be present during

the TC820 signal integrate period. The TC820 does not

perform transient peak detection of the analog input signal.

However, in many cases, such as measuring temperature or

electric motor starting current, the TC820 "acquisition time"

will not be a limitation. If true peak detection is required, a

simple circuit will suffice. See the applications section for

details.

The peak reading function is also available when the

TC820 is in the frequency counter mode. The counter auto-

ranging feature is disabled when peak reading hold is

selected.

10:1 Range Change

The analog input full-scale range can be changed with

the RANGE/FREQ input. Normally, RANGE/FREQ is held

low by an internal pulldown. Connecting this pin to VS+ will

increase the full-scale voltage by a factor of 10. No external

component changes are required.

The RANGE/FREQ input operates by changing the

integrate period. When RANGE/FREQ is connected to VDD,

the signal integration phase of the conversion is reduced by

a factor of 10 (i.e., from 2000 counts to 200 counts).

For the TC820, the 10:1 range change will result in ±4V

full scale. This full-scale range will exceed the common-

mode range of the input buffer when operating from a 9V

battery. If range changing is required for the TC820, a higher

supply voltage can be provided or the input voltage can be

divided by 2 externally.

Frequency Counter

In addition to serving as an analog-to-digital converter,

the TC820 internal counter can also function as a frequency

counter (Figure 4). In the counter mode, pulses at the

RANGE/FREQ input will be counted and displayed.

The frequency counter derives its time base from the

clock oscillator. The counter time base is:

tCOUNT =

fOSC

40,000

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