AME811 Datasheet, PDF (11/15 Pages) Analog Microelectronics – 3-1/2 Digit A/D Converter - Low Power With HOLD And Differential Reference Inputs

Analog Microelectronics, Inc.
AME811/AME811A/AME811R
3-1/2 Digit A/D Converter - Low Power
With HOLD And Differential Reference Inputs
Integrating Capacitor (Cint)
Cint should be selected to maximize the integrator out-
put voltage swing without causing output saturation. A
± 2V full-scale integrator output swing is recommended
if “ANALOG COMMON” is used as signal reference.
For 3 readings/second (fosc = 48 KHz) a 0.22 µF value
is suggested. If a different oscillator frequency is used,
Cint must be changed in inverse proportion to maintain
the nominal ± 2V integrator swing. An exact expres-
sion for Cint is:
Cint = [(4000)(1/fosc)(Vfs/Rint)] / Vint
where:
fosc= Oscillator clock frequency
Vfs = Full-scale input voltage
Rint = Integrating resistor
Vint = Desired full-scale integrator output swing
Cint must have low dielectric absorption to minimize
rollover error. A polypropylene capacitor is recom-
mended.
Integrating Resistor (Rint)
The input buffer amplifier and integrator both have a
class A output stage with 100 µA quiescent current.
The integrator and buffer can supply 20 µA drive cur-
rents with negligible linearity errors. Rint is chosen to
keep the output stage in the linear region. For a 200mV
full-scale, it is 47KΩ; 2.0V full-scale requires 470KΩ.
Summary of component selection:
F u ll sc a le
Caz
R in t
C int
V re f
200.0m V
0.47_µF
47 K _Ω
0.22_µF
100.0m V
2.000V
0.047_µF
470 K _Ω
0.22 _µF
1.000V
Note: fosc = 48 KHz
Oscillator Components
R-C Oscillator
A 100 KΩ Rosc is recommended for all frequencies.
Cosc is selected by using the equation:
fosc = 0.45/(RC)
For fosc of 48KHz, Cosc is 100pF nominally.
To achieve maximum line noise rejection, the signal-
integrate period should be a multiple of line period. The
optimum oscillator frequencies for 60 Hz and 50 Hz
rejection are listed as follows:
For 60 Hz rejection:
40KHz, 48KHz, 60KHz etc.
For 50 Hz rejection:
40KHz, 50KHz, 66-2/3KHz etc.
Reference Voltage Selection
A full-scale reading (2000 counts) requires the input
signal be twice the reference voltage.
F u ll-S ca le V o lta g e
200.0 m V
2.000 V
V re f
100.0 m V
1.000 V
In some applications a scale factor other than unity may
exist between a transducer output voltage and the re-
quired digital reading. Assume, for example, a pres-
sure transducer output is 600 mV for 2000 Ib/in². Rather
than dividing the input voltage by three the reference
voltage should be set to 300 mV. This permits the trans-
ducer input to be used directly. The integrator resistor
would be 120KΩ. In some temperature and weighting
system with variable tare, the offset reading can be
generated by connecting the voltage transducer be-
tween INHI and COMMON and the variable offset volt-
age between COMMON and INLO.
Low Battery Flag (LB)
The low battery flag is set when the supply voltage (V+
to V-) is lower than seven volts, typical. Once the LB is
set, the waveform of the LB will be out of phase with
the BP (Back Plane) to turn on a low battery annuncia-
tor for AME811.
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