TC818 datasheet(10/24 Pages) MICROCHIP

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TC818A

DS21475B-page 10

3.5

Voltage Measurement

Resistive dividers are automatically changed to provide

in range readings for 200mV to 2000V full scale read-

ings (Figure 3-1). The input resistance is set by exter-

nal resistors R14/R13. The divider leg resistors are R9-

R12. The divider leg resistors give a 200mV signal VI

(Pin 44) for full scale voltages from 200mV to 2000V.

For applications which do not require a 10m

Ω input

impedance, the divider network impedances may be

lowered. This will reduce voltage offset errors induced

by switch leakage currents.

3.6

Current Measurement

The TC818A measures current only under manual

range operation. The two user selectable, full scale

ranges are: 20mA and 200mA. Select the current Mea-

surement mode by holding the I input (Pin 63) LOW at

digital ground potential. The OHM input (Pin 2) is left

floating or tied to the positive supply.

Two ranges are possible. The 20mA full scale range is

selected by connecting the 20mA input (Pin 3) to digital

ground. If left floating, the 200mA full scale range is

selected.

External current-to-voltage conversion resistors are

used at the current input (II input, Pin 45). For 20mA

measurements, a 10

Ω resistor is used. The 200mA

range requires a 1

Ω resistor; full scale is 200mV.

Printed circuit board trace resistance between analog

common and R16 must be minimized. In the 200mA

range, for example, a 0.05 trace resistance will cause a

5% current-to-voltage conversion error at II (Pin 45).

The extended resolution measurement option operates

during current measurements.

To minimize rollover error, the potential difference

between analog common (COM, Pin 29) and system

commonmust beminimized.

3.7

AC-to-DC Measurements

In voltage and current measurements, the TC818A can

be configured for AC measurements. An on-chip oper-

ational amplifier and external rectifier components per-

form the AC-to-DC conversion.

When power is first applied, the TC818A enters the DC

Measurement mode. For AC measurements (current or

voltage), AC/DC (Pin 62) is momentarily brought LOW

to digital ground potential; the TC818A sets up for AC

measurements, and the AC liquid crystal display

annunciator activates. Toggling AC/DC LOW again

returns the TC818A to DC operation.

If the Manual Operating mode has been selected, tog-

gling AC/DC resets the TC818A back to the Auto-

Range mode. In Manual mode operation, AC or DC

should be selected first, then the desired range.

The minimum AC full scale voltage range is 2V. The DC

full scale minimum voltage is 200mV.

AC current measurements are available on the 20mA

and 200mA full scale current ranges.

3.8

Conversion Timing

The TC818A analog-to-digital converter uses the con-

ventional dual slope integrating conversion technique,

with an added phase that automatically eliminates zero

offset errors. The TC818A gives a zero reading with a

0 volt input.

The TC818A is designed to operate with a low cost,

readily available 32.768kHz crystal. It serves as a time-

base oscillator crystal in many digital clocks. (See

External Crystal Sources.)

The external clock is divided by two. The internal clock

frequency is 16.348kHz, giving a clock period of

61.04µsec. The total conversion — auto-zero phase,

signal integrate and reference de-integrate — requires

8000 clock periods or 488.3msec. There are approxi-

mately two complete conversions per second.

The integration time is fixed at 1638.5 clock periods or

100msec. This gives rejection of 50/60Hz AC line

noise.

The maximum reference de-integrate time, represent-

ing a full scale analog input, is 3000 clock periods

(183.1msec) during manual extended resolution oper-

ation. The 3000 counts are available in Manual mode,

extended resolution operation only. In Auto-Ranging

mode, the maximum de-integrate time is 2000 clock

periods. The 1000 clock periods are added to the auto-

zero phase. An auto-ranging or manual conversion

takes 8000 clock periods. After a zero crossing is

detected in the Reference De-integrate mode, the auto-

zero phase is entered. Figure 3-4 shows the basic

TC818A timing relationships.

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