The circuit in Determine 1 converts the enter DC voltage right into a pulse practice. The interval of the pulses is proportional to the enter voltage with a 50% obligation cycle and a nonlinearity error of 0.01%. The utmost conversion time is lower than 5 ms.
Determine 1 The circuit makes use of an integrator and a Schmitt set off with variable hysteresis to transform a DC voltage right into a pulse practice the place the interval of the pulses is proportional to the enter voltage.
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The circuit is made of 4 sections. The op-amp IC1 and resistors R1 to R5 create two reference voltages for the integrator.
The integrator, constructed with IC2, RINT, and CINT, generates two linear ramps. Swap S1 adjustments the route of the present going to the integrating capacitor; in flip, this adjustments the route of the linear ramps. The remainder of the circuit is a Schmitt set off with variable hysteresis. The low journey level VLO is mounted, and the excessive journey level VHI is variable (the enter voltage VIN is available in there).
The sign coming from the integrator sweeps between the 2 journey factors of the set off at an equal charge and in reverse instructions. Since R4 = R5, the obligation cycle is 50% and the switch perform is as follows:
To start out oscillations, the next relation have to be glad when the circuit will get energy:
Determine 2 exhibits that the switch perform of the circuit is completely linear (the R² issue equals unity). In actuality, there are slight deviations across the straight line; with respect to the span of the output interval, these deviations don’t exceed ± 0.01%. The slope of the road will be adjusted to 1000 µs/V by R2, and the offset will be simply cancelled by the microcontroller (µC).
Determine 2 The switch perform of the circuit in Determine 1. It is rather linear and will be simply adjusted through R2.
Determine 1 exhibits that the µC converts interval T right into a quantity by filling the interval with clock pulses of frequency fCLK = 1 MHz. It additionally provides 50 to the end result to cancel the offset. The vary of the obtained numbers is from 200 to 4800, i.e., the decision is 1 rely per mV.
Decision will be simply elevated by an element of 10 by setting the clock frequency to 10 MHz. The nice factor is that the nonlinearity error and conversion time stay the identical, which isn’t potential for the voltage-to-frequency converters (VFCs). Right here is an instance.
Assume {that a} voltage-to-period converter (VPC) generates pulse intervals T = 5 ms at a full-scale enter of 5 V. Filling the interval with 1 MHz clock pulses produces quite a few 5000 (N = T * fCLK). The conversion time is 5 ms, which is the longest for this converter. As we already know, the nonlinearity is 0.01%.
Now contemplate a VFC which produces a frequency f = 5 kHz at a 5-V enter. To get the variety of 5000, this sign have to be gated by a sign that’s 1 second lengthy (N = tG * f). Gate time is the conversion time.
The nonlinearity on this case is 0.002 % (see References), which is 5 instances higher than VPC’s nonlinearity. Nevertheless, conversion time is 200 instances longer (1 s vs. 5 ms). To get the identical variety of pulses N for a similar conversion time because the VPC, the full-scale frequency of the VFC should go as much as 1 MHz. Nevertheless, nonlinearity at 1 MHz is 0.1%, ten instances worse than VPC’s nonlinearity.
The distinction turns into extra pronounced when the specified quantity is moved as much as 50,000. Utilizing the identical evaluation, it turns into clear that the VPC can do the job 10 instances quicker with 10 instances higher linearity than the VFCs. A further benefit of the VPC is the decrease price.
If you happen to plan to make use of the circuit, take note of the integrating capacitor. As CINT participates within the switch perform, it must be fastidiously chosen by way of tolerance, temperature stability, and dielectric materials.
Jordan Dimitrov is {an electrical} engineer & PhD with 30 years of expertise. At present, he teaches electrical and electronics programs at a Toronto group faculty.
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References:
- AD650 voltage-to-frequency and frequency-to-voltage converter. Information sheet from Analog Units; www.analog.com
- VFC320 voltage-to-frequency and frequency-to-voltage converter. Information sheet from Burr-Brown; www.ti.com
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