|
Here is an example of an integrator type digital volt meter (DVM). Please allow time for the counters to increment to the correct number. It can read a voltage up to around 10V with an accuracy of about 200mV. (The circuit is somewhat glitchy and fine-tuned, but the concepts are still good)
DVM Series:
Ramp Type https://everycircuit.com/circuit/4592385207304192
Dual Slope Integrating Type https://everycircuit.com/circuit/6675184624599040
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
INSTRUCTIONS:
The voltage source on the left provides a voltage to be measured. The 7-segment displays on the right show the result. When the counters stop, then the measurement is complete (this may take some time). If you want to measure a different voltage, stop the simulation, adjust the voltage source, and restart the simulation. This example will read a voltage of 6.0V
GENERAL:
The basics are that this circuit converts a voltage level to a frequency. This is done using an integrator opamp. The pulses of the frequency are counted during a specific time period. The result is displayed. The higher the voltage, the higher the frequency, hence the more counts it will have, resulting in a higher number displayed.
DETAILS:
First, the input voltage is passed through a subtractor opamp (orange waveform). Then, it is integrated using opamp, resistor, and capacitor to form a ramp voltage (blue waveform) whose slope is proportional to the input voltage.
Vramp = Vin × t / (R × C)
The ramp voltage is compared to a reference of 5V. The time it takes for the output to reach 5V is given by
T1 = (Vref / Vin) × R × C
In this case, T1 = (5/6) × 40k × 100n = 3.3ms
(Note that as Vin increases, T1 decreases or frequency increases. This is where the voltage to frequency conversion happens). When the output reaches a reference of 5V, a pulse is sent to the 555 timer. The 555 timer acts as a monostable multi-vibrator to generate a pulse (green waveform) for a set period of
T2 = 1.1 × R × C
In this case, T2 = 1.1 × 560 × 33n = 20us
(Note how T2 << T1. This is important, so the overall frequency is dependent mostly upon T1, which is modified by Vin). This pulse does 2 things. First, it increments the counters. Second, it discharges the integrator capacitor, so this cycle can repeat. This continues to happen for a period of time set by the clock (red waveform). The number of counts while the clock signal is high (Ts) is
N = Ts / T1
In this case, N = 200ms / 3.3ms = 60
This is correct because each number translates to 0.1V. (Note clock period (Pclk) is the sample rate. The measurement is taken while the clock is high (Ts). Ts = 80% × Pclk).
|
