Analog-to-digital
Easy conversion
A look at how to implement an
analog-to-digital conversion in a
low bandwidth application
Implementing an
analogue-to-digital
conversion in a circuit is
one of the most common
tasks facing designers
and one that can be done
in various ways. But for
many simple and low
bandwidth applications,
such as a DC voltmeter
for example, the goal is to
keep the cost of the
implementation low but
still obtain a high
resolution for the
analogue-to-digital
conversion.
A simplified schematic of such a circuit is
shown in Fig. 1. There are two input
voltages connected one at a time to op
amp U1. Vref is the fixed reference voltage
used in calibration and Vmeas is the
unknown voltage to be converted. Resistor
R1 and capacitor C1 form a charging
circuit used to convert input voltage to
time. The existence of U1 in the circuit
removes the logarithmic characteristic
that would occur if the input voltage is
directly applied to R1 and C1.
This circuit uses a PIC16F5X
microcontroller from Microchip to control
the U1 operation by turning the four
switches (S1 to S4) on and off.
Additionally, the microcontroller measures
the time and calculates the digital
representation of the unknown input
voltage.
ELE Times | 44 | November, 2016
Fig. 1: Circuit diagram for an analogue-todigital circuit
The circuit can also be used as a current
mode A-D converter. In this case, the input
voltage to the current converter is not
needed and the reference current and
input current are both routed via analogue