3-180

TELCOM SEMICONDUCTOR, INC.

TC826

A/D CONVERTER WITH

BAR GRAPH DISPLAY OUTPUT

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 7.

40

39

2

1

0

–1

–2

1

2

3

39 39.5 40 40.5

–0.5

0.5

OVERRANGE

INDICATION

ANALOG INPUT

(X

VFS

40

)

Figure 7. TC826 Transfer Function

BAR/DOT Input (Pin 61)

The BAR/DOT input allows the user to select the display

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.

HOLD Input (Pin 62)

The TC826 data ouput latches are not updated at the

end of each conversion if HOLD is tied to the negative supply

(Pin 12) through a 1 M

Ωresistor.TheLCDdisplaycontinously

displays the previous conversion results.

The HOLD pin is normally pulled high by an internal pull-

up.

TEST Input (Pin 63)

The TC826 enters a test mode with the TEST input

connected to the negative supply (Pin 12). The connection

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

overrange annuniciator flash on and off at 1/4 the conversion

rate. The polarity annunciator (POL–) segment will be on but

not flashing

Overrange 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

overrange annunciator, OR, will flash ON and OFF. The

flash rate is on fourth the conversion rate (FOSC/2560).

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

segment drivers are off.

Polarity Indication (POL– Pin 60)

The TC826 converts and displays data for positive and

negative input signals. The POL– LCD segment driver (Pin

60) is active for negative signals.

Oscillator Operation

The TC826 external oscillator frequency, FOSC, is set

by resistor ROSC connected between pins 13 and 14. The

oscillator frequency versus resistance curve is shown in

Figure 8.

20

18

16

14

12

10

8

6

4

2

0

50

40

30

20

10

0

0

2468 10 12 14 16 18 20

TA = 25°C

VDDTO VSS= 9V

ROSC (X 100kΩ)

Figure 8. Oscillator Frequency vs. ROSC

FOSC is divided by 32 to provide an internal system

clock, FYSY. Each conversion requires 80 internal clock

cycles. The internal system clock is divided by 8 to provide

the LCD backplane drive frequency. The display flash rate

during an input out-of-range signal is set by dividing FSYS

by 320.

The internal oscillator may be bypassed by driving OSCI

(Pin 13) with an external signal generator. OSC2 (Pin 14)

should be left unconnected.

supply operation (Figure 9). In dual supply operation the