參數(shù)資料
型號(hào)： TC818
廠商： Microchip Technology Inc.
英文描述： Auto-Ranging Analog-to-Digital Converter with 3-1/2 Digit Display
中文描述： 自動(dòng)測(cè)距模擬到數(shù)字轉(zhuǎn)換器與數(shù)字顯示3-1/2
文件頁(yè)數(shù)： 10/24頁(yè)
文件大?。?/td> 583K
代理商： TC818
TC818A
DS21475B-page 10
2002 Microchip Technology Inc.
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 V
I
(Pin 44) for full scale voltages from 200mV to 2000V.
For applications which do not require a 10m
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 (I
I
input, Pin 45). For 20mA
measurements, a 10
resistor is used. The 200mA
range requires a 1
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 I
I
(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
common must be minimized.
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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