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Converters

Reference Book: Electrical Measurement &

Instrumentation, By-A.K. Sawhney,Chapter: 26

Prepared By: Md. Ahsanul Hoque, Lecturer, IIUC

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Successive Approximation

(Potentiometric A/D conversion-

continued )

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Voltage to Time A/D converter (Ramp type)

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Figure: 26.111 Voltage to Time A/D converter (Ramp type)

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Voltage to Time A/D converter (Ramp type)

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Voltage to Frequency Converter (Integrating Type)

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Dual slope integration A/D converter

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A classical dual-slope converter is shown at the drawing

A current, proportional to the input voltage, charges a capacitor for a fixed time

interval Tcharge. At the end of this interval the device resets its counter and applies anopposite-polarity (negative) reference voltage to the integrator input. With this opposite-

polarity signal applied the cap is discharged by a constant current until the voltage at the

output of the integrator reaches zero again. The timeTdischarge is proportional to the input

voltage level and used to enable a counter. The final count provides the digital output,

corresponding to the input level.

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Note that even the clock frequency does not have to have high stability, because both

ramp-up and ramp down time are measured with the same clock.If the clock slows down

10%, the initial ramp will go 10% higher than normal, requiring 10% longer ramp down

time resulting in the same final count. Only the discharge current produced by precise

Vref has to be of high stability.

Integrating ADCs are extremely slow devices with low input bandwidths. But their ability

to reject high-frequency noise and fixed low frequencies such as 50Hz or 60Hz makes

them useful in noisy industrial environments and applications . Provide 10-18 bit

resolution. A coversion time for a medium speed 12 bit integrating ADC is about 20mS.

This type of ADC is most commonnly used in multi-meters.

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Sample & Hold operation

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Purpose The sample and hold circuits are essentially used in linear systems. In some kinds of

analog-to-digital converters, the input is often compared to a voltage generated

internally from a digital-to-analog converter (D-A-C). The circuit tries a series ofvalues and stops converting once the voltages are "the same" within some defined

error margin. If the input value was permitted to change during this comparison

process, the resulting conversion would be inaccurate and possibly completely

unrelated to the true input value. Such successive approximation converters will

often incorporate internal sample and hold circuitry. In addition, sample and hold

circuits are often used when multiple samples need to be measured at the sametime. Each value is sampled and held, using a common sample clock.