TC826 datasheet(11/18 Pages) MICROCHIP

11 / 18 page

DS21477B-page 11

TC826

TABLE 4-2:

SUGGESTED COMPONENT

VALUES

4.5

Differential Signal Inputs

The TC826 is designed with true differential inputs and

accepts input signals within the input stage Common

mode voltage range (VCM). The typical range is V+ -1

to V- +1V. Common mode voltages are removed from

the system when the TC826 operates from a battery or

floating power source (isolated from measured sys-

tem) and -IN is connected to analog common (VCOM).

In systems where Common mode rejection ratio mini-

mizes error. Common mode voltages do, however,

affect the integrator output level. Integrator output sat-

uration must be prevented. A worse case condition

exists if a large positive VCM exists in conjunction with

a full scale negative differential signal. The negative

signal drives the integrator output positive along with

VCM. For such applications, the integrator output swing

can be reduced below the recommended 2V full scale

swing. The integrator output will swing within 0.3V of

VDD or VSS without increased linearity error.

4.6

Digital Section

The TC826 contains all the segment drivers necessary

to drive a liquid crystal display (LCD). An LCD back-

plane driver is included. The backplane frequency is

the external clock frequency divided by 256. A 430k

Ω

OSC gets the backplane frequency to approximately

55Hz, with a 5V nominal amplitude. When a segment

driver is in phase with the backplane signal, the seg-

ment is ‘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 drive, -POL, is ‘ON’ for negative

analog inputs. If +IN and -IN are reversed, this indicator

would reverse. The TC826 transfer function is shown in

Figure 4-4.

FIGURE 4-4:

TRANSFER FUNCTION

4.7

BAR/DOT Input (Pin 61)

The BAR/DOT input allows the user to select the dis-

play format. The TC826 powers up in the BAR mode.

Select the DOT display format by connecting BAR/DOT

to the negative supply (Pin 12) through a 1M

Ω resistor.

4.8

HOLD Input (Pin 62)

The TC826 data output latches are not updated at the

end of each conversion if HOLD is tied to the negative

supply (Pin 12) through a 1M

Ω resistor. The LCD dis-

play continuously displays the previous conversion

results.

The HOLD pin is normally pulled high by an internal

pull-up.

4.9

TEST Input (Pin 63)

The TC826 enters a Test mode with the TEST input

connected to the negative supply (Pin 12). The connec-

tion must be made through a 1M

Ω resistor. The TEST

input is normally internally pulled high. A low input sets

the output data latch to all ones. The BAR Display

mode is set. The 41 LCD output segments (zero plus

40 data segments) and over range annunciator flash on

and off at 1/4 the conversion rate. The polarity annun-

ciator (POL-) segment will be on, but not flashing.

4.10

Over Range Display Operation

(OR, Pin 59)

An out-of-range input signal will be indicated on the

LCD display by the OR annunciator driver (Pin 59)

becoming active.

In the BAR display format, the 41 bar segments and the

over range annunciator, OR, will flash ON and OFF. The

flashrateisonfourth theconversionrate(FOSC/2560).

In the DOT Display mode, OR flashes and all other data

segment drivers are off.

Comp.

Full Scale

VREF ≈ 1V

2mV

Full Scale

VREF ≈ 100V

20mV

Full Scale

VREF ≈ 10V

RINT

Ω

200k

Ω

20k

Ω

CINT

0.033

µF0.033µF0.033µF

CREF

µF1µF1µF

CAZ

0.068

µF0.068µF1.14µF

ROSC

430k

Ω

430k

Ω

430k

Ω

Note: Approximately 5 conversions/second.

-1

-2

39 39.5 40 40.5

-0.5

0.5

Over Range

Indication

Analog Input

VFS

)

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