Single Slope (RAMP) Converter

(a) Description of the Block Diagram:

The block diagram of a single slope converter type voltmeter is shown in Figure. We find the capacitor, constant current source, and the comparator in the block diagram. An oscillator, gate, counter, decoder and driver for the display are arranged as shown in the block diagram. Two switches S1 and S2 are used.
Block Diagram of Single Slope Digital Voltmeter

(b) Working:

The voltage comparator derives its input from the charge developed across the capacitor. The capacitor is charged from a constant current source. The other input to voltage comparator is from the unknown voltage. In the block diagram they are shown as Ec (voltage across the capacitor) and Ex (unknown voltage).

The voltage comparator converts the analog information to digital signal, at its output. If the voltage at non-inverting input of the comparator is greater than the input at inverting input, the output will be high i.e. ' 1 '. If the voltage at non-inverting input is lower than inverting input the output of the comparator will be low i.e. ‘0’.

The output of the comparator is given to one of the three inputs of the AND gate. The output of a known frequency oscillator is applied to the second input of the gate. To the third input of the gate a switch is connected S2. S2 is an SPDT switch and can apply either a high level signal i.e. a logic 1 or a low level signal i.e. a logic '0' to the third input of the gate.

When the output of the comparator is ‘1’ and switch S2 is at high level, the oscillator signal is allowed to the counter and associated circuits. If the output of the comparator is '0', and S2 is applying a low level signal the signal from the oscillator cannot reach the counter.

Initially we require the counter to be reset to zero. To effect resetting of the counter the switch S1, is to be closed shorting the capacitor and keeping S2, at zero.

The measurement cycle can be started by opening the switch S1, and keeping switch S2 to logic ‘1' position. Now the situation at the input of the comparator is that the unknown voltage is greater than the voltage across the capacitor. As unknown voltage is at the non-inverting input and the capacitor voltage is at inverting input of the comparator the output of the comparator will be logic ‘1'. As S2, is at logic ‘1’ and the comparator's output is logic ‘1', the gate allows the oscillator output to reach the counter.

Hence the counter accumulates the counts. While count is going on, the voltage across the capacitor goes on increasing. This will be presenting an increasing voltage at the inverting input of the comparator. When this voltage of the capacitor increases and equals the value of the unknown voltage the comparator changes its output at the instant the capacitor voltage is just more than the input voltage. That is the output of the comparator changes from ‘1' to '0'. This prevents the signal flow from the oscillator through the gate to the counter circuit.

The counts accumulated in the counter represent the time the gate was open. This time is directly and linearly proportional to the unknown voltage. Finally we have to modify the charge rate of the constant current source or to adjust the oscillator frequency such that the accumulated value in the counter is not only proportional to the unknown voltage, but also a voltage of 1000 V produces exactly 1000 counts. If this is done the display gives the input voltage.

(c) Limitations of the Single Slope Converter Type Digital Volt Meter:

1. It measures only single polarity voltages.
2. Additional circuits are needed to detect input voltages greater than maximum charging voltage of the capacitor. That is called over ranging circuit.
3. It suffers from long term errors.
4. It is effected by the frequency drift of the oscillator.
5. It is effected by the drift of the output current of constant current source.
6. The accuracy is dependent on the stability of the capacitor.
7. The accuracy is dependent on the stability of differential voltage required to trip the comparator.
8. The converter is effected by the noise pulses of the input voltage.
9. The accuracy depends on the linearity of charging of the capacitor.

(d) Applications of Single Slope Converter Type Digital Volt Meters:

In spite of the limitations single slope converter type of digital voltmeters are used in low cost Digital Volt Meters.

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