7

TC811

TC811-7 11/5/96

© 2001 Microchip Technology Inc.

DS21472A

3-1/2 Digit Analog-To-Digital Converter with

Hold and Differential Reference Inputs

power dissipation, and improve the overall performance.

(see Oscillator Components)

Digital Section

The TC811 contains all the segment drivers necessary

to directly drive a 3-1/2 digit liquid crystal display (LCD). An

LCD backplane driver is included. The backplane frequency

is the external clock frequency divided by 800. For three

conversions/second the backplane frequency is 60Hz with

a 5V nominal amplitude. When a segment driver is in phase

with the backplane signal the segment of “OFF”. An out of

phase segment drive signal causes the segment to be “ON”

or visible. This AC drive configuration results in negligible

DC voltage across each LCD segment. This insures long

LCD display life. The polarity segment driver is “ON” for

indicator would reverse.

TEST Function (TEST)

On the TC811, when TEST is pulled to a logical “HIGH”,

all segments are turned “ON”. The display will read “-1888”.

During this mode the LCD segments have a constant DC

voltage impressed. Do not leave the display in this mode for

more than several minutes. LCD displays may be destroyed

if operated with DC levels for extended periods.

The display FONT and segment drive assignment are

shown in Figure 5.

Figure 5. Display FONT and Segment Assignment

DISPLAY FONT

1000's

100's

10's

1's

Polarity is determined at the end of signal integration

phase. The sign bit is a “true polarity” indication in that

signals less than 1 LSB are correctly determined. This

allows precision null detection which is limited only by device

noise and Auto Zero residual offsets.

Reference Integrate (Deintegrate) Cycle

The reference capacitor, which was charged during the

Auto Zero cycle, is connected to the input of the integrating

amplifier. The internal sign logic insures that the polarity of

the reference voltage is always connected in the phase

which is opposite to that of the input voltage. This causes the

integrator to ramp back to zero at a constant rate which is

determined by the reference potential.

amplifier to reach zero is directly proportional to the ampli-

tude of the voltage that was put on the integrating capacitor

The digital reading displayed Is:

Digital Count = 1000

The oscillator frequency is divided by 4 prior to clocking

the internal decade counters. The four phase measurement

cycle takes a total of 4000 counts or 16000 clock pulses. The

4000 count cycle is independent of input signal magnitude

or polarity.

Each phase of the measurement cycle has the following

length:

1) Auto Zero: 300 to 2900 Counts

2) Signal Integrate: 1000 Counts

This time period is fixed. The integration period is:

= 1000 Counts

3) Reference Integrate: 0 to 2000 Counts

4) Integrator Output Zero: 11 to 640 Counts

The TC811 can replace the ICL7106/26/36 in circuits

which require both the hold function and a differential

reference. The TC811 offers a greatly improved internal

reference temperature coefficient, which can often eliminate

the need for an external reference. Some minor component

changes are required to upgrade existing designs, reduce

+

–

HOLD Reading Input (HLDR)

When HLDR is at a logic “HI” the latch will not be

updated. Conversions will continue but will not be updated

until HLDR is returned to “LOW”. To continuously update the

display, connect HLDR to ground or leave it open. This input

is CMOS compatible and has an internal resistance of 70k

Ω

(typical) tied to TEST.

4000