ES5108 3 1/2 Digit
ADC with serial output
CYRUSTEK CO.
2 Reference Voltage
For a 1000 counts reading, the input signal must be equal to the reference voltage. As a result, it requires the input signal
be twice the reference voltage for a 2000 counts full-scale reading. Thus, for the 200.0mV and 2.000V full-scale, the
reference voltage should equal 100.0mV and 1.000V, In some applications the full-scale input voltage my be other than
200mV or 2V, but 600mV. For example, the reference voltage should be set to 300mV and the input signal can be used
directly without being divided.
The differential reference can be used during the measurement of resistor by the ratiometric method and when a digital
reading of zero is desired for
0. A compensating offset voltage can be applied between COMMON and VIN- and
the voltage of being measured is connected between COMMON and VIN+.
3 System Timing
The oscillator frequency is divided by four prior to clocking the internal decade counters. The signal integrate takes a
fixed 1000 counts time period which is equal to 4000 clock Pulses. The backplane drive signal is derived from dividing
clock frequency by 240. To make a maximum rejection of line frequency(60Hz or 50Hz)noise pickup, the signal integrate
period should be a multiple of the line frequency period. For 60Hz-noise rejection, oscillator frequencies of 120KHz,
80KHz, 60KHz, 48KHz, 40KHz, etc. should be selected. For 50Hz rejection, oscillator frequencies of 100KHz, 50KHz,
40KHz, etc. would be suitable.
For all ranges of frequency
48KHz clock (3reading/second),
should be 100K
=100PF.
,
is selected from the approximate equation f 0.45/RC. For
4 Integrating Resistor
The input buffer amplifier and integrator both have class A output stage with 100 A of quiescent current and can supply
20 A drive currents with negligible linearity errors. The integrating resistor should be chosen to remain in the output
stage linear drive region. It should be noticed that the integrating resistor should not be so large such that the leakage
currents of printed circuit board will induce errors. For a 200mV full-scale the recommended integrating resistor value is
47K and for 2V full-scale is 470K .
5 Integrating Capacitor
The integrating capacitor should be selected to maximize integrator output voltage swing without causing output satura-
tion. If the analog COMMON is used as voltage reference, a 2V full-scale integrator output swing is satisfactory. For
3 readings/second (48KHz clock) a 0.22 F value of
is suggested. When different oscillator frequencies are used,
must be changed in inverse proportion to maintain the nominal 2V integrator swing.
The integrating capacitor should have low dielectric absorption to minimize roll-over error. An inexpensive polypropy-
lene capacitor will work well.
6 Auto-Zero Capacitor
The auto-zero capacitor size has some influence on system noise. A 0.47 F capacitor is recommended for 200mV full
scale where noise is very important. A 0.047 F capacitor is adequate for 2V full-scale applications. A mylar type
dielectric capacitor is adequate.
7 Reference Voltage Capacitor
When VIN- is tied to analog COMMON, a 0.1 F capacitor adequate to be the reference capacitor. If a large common-mod
voltage exists and the application requires a 200mV full scale, a larger value is required to prevent roll-over error. A 1.0 F
capacitor will hold the roll-over error to 0.5 count.
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November 23, 2006
CYRUSTEK [ Cyrustek corporation ]
