A/D Converters

An analog-to-digital converter (abbreviated ADC or  A/D )  is a device that transforms a signal from analog (continuous) to digital (discrete) form.

The reverse operation is performed by a digital – to – analog – converter  (DAC).
Analog signals are directly measurable.
Digital signals only have two states.
Microprocessors can only perform processing on digitized signals.
ADC is a link between the analog world  and the digital world of signal processing and data handling.

Application of ADC:

digital voltmeters, cell phone, thermocouples, and digital oscilloscope.

Accuracy of A/D Conversion:

  • Increasing the resolution which improves the accuracy in measuring the amplitude of the analog signal.
  • Increasing the sampling rate which increases the maximum frequency that can be measured.

Resolution:

The smallest change in analog signal that will result in a change in the digital output.

Sampling rate:

The  faithful reproduction of the converted signal  is only possible if the sampling rate is higher than twice the highest frequency of the signal.(Shannon-Nyquist-theorem)

Signal-to-Noise Ratio (SNR) is the ratio of the output signal amplitude to the output noise level.
It can easily calculated:     6.02 x  n + 1.76 dB.

n is the number of bits used on the ADC. The higher the SNR the better.

Three different input structure types exist: Single-Ended, Pseudo-Differential and Fully-Differential.

Single-ended inputs are generally sufficient for most applications.

fully-differential inputs: for maximum noise rejection  at dynamics signals

pseudo-differential inputs  is typical used in  measuring sensors.

Main Types of A/D Converters:

  • Successive Approximation A/D Converter
  • Delta-Sigma A/D Converter
  • Flash – A/D ConverterSAR:  8- to 16-bit range and can have sample speeds up to 1 MSPS.

One major benefit of a SAR converter is its ability to be connected to multiplexed inputs at a high data acquisition rate. The input is sampled and held on an internal capacitor, and this charge is converted to a digital output code using the successive approximation routine. Since this charge is held throughout the conversion time, only the initial sample and hold period or acquisition time is of concern to a fast-changing input. The conversion time is the same for all conversions. This makes the SAR converter ideal for many real-time applications: motor control, touch-screen sensing, medical

Delta-Sigma:

the input signal is oversampled and then filtered by a digital/decimator filter. Due to the  oversampling a delta – sigma is slow and so prefered for low bandwith and no real- time applications.
The typical  applications are  resulted due of its high resolution: video, imaging,audio.

Flash ADC:

has a bank of comparators. 2N-1 for (For N bit Flash A/D)

Direct conversion is very fast  but expensive, capable of  gigahertz  sampling rates but low resolution.

Application:video, radar, wideband communications.

Operational Amplifiers

Amplifiers are widely used devices as they have the ability to amplify a relatively small input signal, for example from a   Sensor or loudspeaker.

Amplifiers can be thought  as a simple box or block containing the amplifying device, which has two input terminals and two output terminals with the output signal being much greater than that of the input signal =>  it has been “Amplified”.

An ideal signal amplifier has some main parameters.

No matter how complicated an amplifier circuit is, a general amplifier model can still be used to show the relationship of these parameters.

The power  gain or power level of the amplifier can also be expressed in Decibels, (dB). The Bel is a logarithmic unit (base 10) of measurement that has no units. Since the Bel is too large a unit of measure, it is prefixed with deci making it Decibels instead with one decibel being one tenth (1/10th) of a Bel. To calculate the gain of the amplifier in Decibels or dB, we can use the following expressions.

Voltage Gain in dB:    av  =  20 log Av

The bandwith  of an amplifier is the range of frequencies for which the amplifier gives well  performance.

The gain–bandwidth product ( GBWP) is  the product of  the amplifier’s bandwith and the  gain at which the bandwidth is measured. This is the frequency range in which the amplifier amplified linear and stable.

Slew rate, SR, is the rate of change in the output voltage caused by a step input. Its units are V/ìs or V/ms. As the frequency gets higher and higher the output  becomes slew rate limited and can not respond quickly enough to maintain the specified output voltage swing.

INPUT OFFSET VOLTAGE:
The input offset voltage can range from microvolts to millivolts and can be either polarity. Generally, bipolar op amps have lower offset voltages than JFET or CMOS types.
Choosing an amplifier with a low Vio is important for application requiring a high gain in DC.

Rail to rail input/output:
capability  input / output can be taken to positive or negative power supply rail within a few milivolts

Noise:
This is a measure of how much  noice  is introduced in the amplification process. Noise is an undesirable but inevitable product of the electronic devices and components. It is expressed in   The  product of the  value  and the frequency is the result.

Split Supply vs  Dual// Single Supply:
All op amps have two power pins. Single-supply operation requires a little more care than split-supply circuits. A single-supply circuit  connects the op-amp power pins to a positive voltage and ground. This is usefull at  battery operated portable eqipment. In this case you must ensure that signal swings between correct voltages (VCC and GND)

A split power supply consists of a positive supply and an equal and opposite negative supply.This allows a  wider output voltage range and also a significant flexibility when designing circuits.

If you regard  this parameters you will find the best Op-Amp to your application.

Instrumentation Amplifiers abbreviation

Instrumentation amplifiers offer a unique combination of differential inputs, high input impedance, and excellent precision and noise specifications.

An instrumentation amplifier is a type of differential amplifier  that has been outfitted with input buffers, which eliminate the need for input impedance matching and thus make the amplifier particularly suitable for use in measurement.

Although the instrumentation amplifier is usually shown schematically identical to a standard op-amp, the electronic instrumentation amp is almost always internally composed of 3 op-amps. These are arranged so that there is one op-amp to buffer each input (+,−), and one to produce the desired output with adequate impedance matching for the function.

Parameters that define a high quality instrumentation amplifier:

  • high common mode rejection ratio;
  • low offset voltage and offset voltage drift;
  • low input bias and input offset currents;
  • high-value input impedances;
  • low noise
  • low non-linearity
  • adequate bandwidth.

Examples of applications:

  • data acquisition from low output transducers;
  • medical instrumentation;
  • current/voltage monitoring;
  • audio applications involving weak audio signals or noisy environments;
  • high-speed signal conditioning for video data acquisition and imaging;
  • high frequency signal amplification in cable RF systems.
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