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This is a simplified example of a dual slope integrating type digital volt meter (DVM). It can read a voltage up to around 10V with an accuracy of about 100mV. Please allow time for the counters to increment to the correct number.
INSTRUCTIONS:
The voltage source in the top left provides a voltage to be measured. The 7-segment displays in the top right show the result of the measured value (first, wait until it counts all the way up to 9.9 and resets, then it will count up to the correct amount). Note that probably some kind of analog switch would be used instead of a relay. This example reads a voltage of 3.3V.
DVM Series:
Ramp Type https://everycircuit.com/circuit/4592385207304192
Integrator Type (voltage to frequency) https://everycircuit.com/circuit/6172260362354688
Successive Approximation Type (hex) https://everycircuit.com/circuit/6044286526947328
Successive Approximation Type (decimal)
https://everycircuit.com/circuit/5334865438048256
Ramp Type (negative volts) https://everycircuit.com/circuit/5763215168110592
Ramp Type (auto ranging) https://everycircuit.com/circuit/6110164975091712
Counter Type ADC https://everycircuit.com/circuit/4509930926899200
Tracking Type ADC https://everycircuit.com/circuit/6418942605918208
Tracking Type ADC Full Version https://everycircuit.com/circuit/4688147021299712
Flash Type ADC https://everycircuit.com/circuit/4806466994962432
Pipeline Type ADC https://everycircuit.com/circuit/5739735974412288
Pipeline Type ADC 2 Bit Stages
https://everycircuit.com/circuit/6081127019970560
GENERAL:
A dual slope integrating DVM works by using two slopes created by integration to calculate the unknown voltage. These 2 slopes create a ramp voltage (blue waveform). The higher the unknown voltage, the higher the ramp will go (first slope). Hence, it will take longer for the ramp to come back down (second slope), and the number displayed will be higher.
DETAILS:
First, the unknown voltage is passed through the left opamp subtractor (orange waveform). Then, it is integrated using the top opamp, resistor, and capacitor, causing a positive going ramp with a slope proportional to the unknown voltage.
Vramp = -Vin × t / (R × C)
In this case, Vramp = 3.3 × t / (3.3k × 1u) = 1000t
This process will continue until the clock, which is allowed through by the opamp comparator and AND gate (red waveform), increments and resets the counters. This will cause the flip-flop to toggle and switch the relay to the reference voltage. The time is determined by the clock period and reset of the counters.
T1 = Pclk × RES
In this case, T1 = 100u × 100 = 0.01s
The ramp will reach its max height now (at T1).
Vmax = -Vin × T1 / (R × C)
In this case, Vmax = 3.3 × 0.01 / (3.3k × 1u) = 10V
Now, the top opamp will integrate the reference voltage. This causes a negative going ramp with a fixed slope proportional to the reference voltage. The ramp starts decreasing from where the positive ramp stopped (Vmax).
Vramp = -Vref × t / (R × C) + Vmax
In this case
Vramp = (10 × t / (3.3k × 1u)) - 10 ~ 10 - 3030t
This process will continue until the ramp goes below 0V at which the comparator opamp will go low and the clock will stop incrementing the counters. This happens when Vramp = 0V and t = T2. The correct value is finally displayed!
0 = -Vref × T2 / (R × C) + Vmax
T2 = Vmax × R × C / Vref (solve for T2)
T2 = -Vin × T1 / Vref (substitute for Vmax)
In this case, T2 = 3.3 × 0.01 / 10 = 0.0033s
The number displayed (N) will be the time the counters increment (T2) divided by the clock period.
N = T2 / Pclk
N = -Vin × RES / Vref (substitute for T2 and T1)
In this case, N = 3.3 × 100 / 10 = 33.
This is correct because each number translates to 0.1V.
The sample signal is the logic clock by the counters (green waveform). It resets the counters and toggles the flip-flop through the OR gates to begin another measurement. It is 20ms
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