Determine 1’s silly-simple voltage-to-time ADC is an train in dynamic vary. Assuming that it’s used with a 10-MHz counter/timer, its decision is roughly 10 µV per rely for inputs round 0 V and 100 mV per rely at 1 kV, and it by no means actually over-ranges. The easy trick that gives this multi-decade measurement span is the inherent logarithmic conduct of RC timing networks.
Right here’s how this one works.
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Determine 1 U1 works with R1, R2, and C1 to logarithmically digitize 0 kV to 1 kV inputs whereas tying up just one microcontroller I/O pin.
Between conversions, U1’s reset pin 4 is held active-low both by the related GPIO pin, or by U1 itself. Utilizing a 555 on this self-resetting mode is uncommon however may be very useful right here. This holds the C1 at zero or very close to, because the Ron of pin 7’s open-drain FET is usually simply 15ohms.
A conversion begins when the I/O pin is programmed for output and pulsed excessive, overriding U1 pin 3 (Out) and releasing the reset as sketched in Determine 2. The I/O pin is then instantly tri-stated and reprogrammed as enter, gating an inside counter/timer peripheral for measurement of the conversion time T.
As a result of U1’s pin 2 (Set off) is held low, the top of reset additionally units pin 3 excessive and releases pin 7, permitting C1 to start to ramp optimistic. When it reaches the 2.048V (Threshold) voltage on pin 5, conversion will full and the time T that was required to take action is the conversion outcome. Arrival of pin 5 on the threshold voltage drives pins 3 (Out) low, thereby each pin 4 (Reset) and GPIO, the latter being the “conversion full” standing bit to the microprocessor. In the meantime, pin 7 is pushed low to discharge C1. This course of completes in about 12 µs, readying the converter for one more cycle.
Determine 2 A single general-purpose tri-state I/O pin serves to each management and measure U1’s time-out. The pin is programmed for output and pulsed optimistic to begin conversion, then tri-stated for timer enter. Conversion time (T) is 10 ms for Vin = 0, reducing to roughly 100 µs for Vin = 1 kV.
T versus Vin is given by the next equation:
T = C1(R1||R2)*Loge ((0.209Vin + 3.234) / (0.209Vin + 1.1919))
= 10.0ms*Loge ((0.209Vin + 3.234) / (0.209Vin + 1.1919))
That is plotted in Determine 3 for Vin = 10 mV to 1kV.
Determine 3 The conversion time (T) in milliseconds for Vin from 0.01 V to 1000 V.
To get well Vin from a T acquisition, do that:
Vin = (3.234 – 1.1919e(T/10.0ms))/(0.209(e(T/10.0ms) – 1))
Lastly, right here’s one thing within the nature of a actuality examine.
You is perhaps questioning why R1 is proven as a collection connection of 4 1M resistors as an alternative of a single 4M part. The reply is the considerably apparent proven fact that 1 kV is a few critical voltage and the considerably much less apparent proven fact that resistors, considerably like capacitors, have voltage rankings. Resistors rated for 1 kV aren’t the same old breed of cat.
And talking of cats, please keep in mind the previous story about what curiosity did to at least one unlucky feline and contemplate that prudent and correct security practices are actually important when selecting to work with voltages of this magnitude.
Me–OW!
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Over 100 submissions have been accepted since his first contribution again in 1974.
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The publish Microvolts to kilovolts in milliseconds with one I/O pin appeared first on EDN.
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