Group: Ursula Emblau, Chris Thompson and Robert Carroll

References: The information on this page was taken exclusively from Terry Sturtevant's lecture notes and Fundamentals of Linear Circuits by Thomas L. Floyd. Images with Figure 13-xx are taken from the text and additional information is available in those sections.

The background for this site was taken from the Datasheet for National Semiconductors DAC0800/DAC0802.

Data sheets for A/D and D/A converters can be downloaded from: National Semiconductor

I. Digital to Analog Converters

A Digital to Analog converter will convert a digital signal in to its analog equivalent.

A. Function

Vo is the analog output voltage

Vref is the reference voltage

N is the total number of bits of digital data permissible

Modes

1. Conversion mode will convert a digital signal to analog.
2. Programmable gain amplifier mode is obtained when the signal is fed directly into Vref. In this mode the output is strictly analog, with no steps and the gain multiplier is in digital form.

B. Parameters

1. The Resolution of a Digital to Analog Converter is defined as,

   

   where N is the total number of bits. This value is usually expressed in percentages.

2. Accuracy is a comparison of the actual output of the converter with the expected output.
3. Settling time is the amount of time it takes the converter to settle within ±1/2 of the least significant bit of its final value after the converter has been given input
4. Power supply sensitivity is defined as,

   

   where Vo is the output voltage, Vs is the source voltage and it is usually expressed in percentages. An ideal converter would have 0% power supply sensitivity.

C. Interfacing

1. If the converter is not buffered it will need an output port. A latch could serve as an output port.
2. When a converter is buffered it already has a built in latch and it could be double buffered.

II. Analog to Digital Converters

Analog to Digital converters will convert an analog signal into its digital equivalent.

A. Function

Vin is the analog input voltage

Vref is the reference voltage

N is the total number of bits of digital data

n is the number of bits in the output

B. Parameters

1. Resolution of an Analog to Digital Converter is defined as the number of bits used to represent each value of the analog signal. As the number of bits increase, so will the accuracy of the conversion, in other words the resolution increases.
2. Conversion time is defined as the time required to convert the analog signal to a digital signal
3. Power supply sensitivity is defined as

   

   where Vo is the output voltage, Vs is the source voltage and it is usually expressed in percentages. An ideal converter would have a 0% power supply sensitivity.

C. Interfacing

A Tri state output is a feature of some digital electronic devices which allows a connector (pin) to either act as a normal output, driving a signal onto a line, or to be "tri stated" - set to a high-impedance ("high Z") condition. This allows other outputs to drive signals onto the line. Often the same connector also functions as an input when its output circuitry is tri stated. Tri-state outputs are typically used for the connection of several digital circuits to a shared bus onto which any one of them may output data for the others to input.

III. Voltage References

Conversions can be very sensitive to supply voltage changes, especially high resolution or many bit conversions, so it may be very important to have a known, and stable supply.  Often immediately available sources, such as hydro outlets, will not provide a precise or stable voltage and a voltage reference device must be used.

A. Single Chip

- Very low sensitivity to power supply (~2ppm/v)

- Very low temperature coefficient (~1ppm/⁰C)

- High initial accuracy (~0.01%)

B. Zener Diode

- Readily available

- Cheap

IV. Sample and Hold Amplifiers

A sample-and-hold amplifier samples an analog input voltage at a certain point in time and retains the sampled voltage for an extended time. This process enables the analog signal to remain constant for the amount of time required to convert it to a digital signal by the Analog to Digital Converter.

A quantization error occurs when there is a change in the analog signal during the conversion time.  This error depends on the frequency and amplitude of the signal and the resolution needed for the digital output.  If the quantization is going to cause error in the digital signal it must be addressed.

A sample-and-hold device, as the name suggests, samples the analog signal when the conversion starts and holds it for the duration of the conversion, eliminating the quantization error.

A. Function

B. Parameters

1. Acquisition time is the time required for the device to reach its final value when the control voltage switches from hold level to sample level.
2. Aperture time is the time needed for the analog switch to fully open after the control voltage switches from its sample level to its hold level.   Aperture time produces a delay in the effective sampling point.
3. Aperture jitter is the uncertainty in aperture time.
4. Droop is the change in voltage of the sample during the hold interval. This is due to the inevitable discharge of the capacitor. It is proportional to the value of the capacitor and is measured in volts per second.
5. Feedthrough is the component of the output voltage that follows the input signal after the analog switch is opened.  The inherent capacitance from the input to output of the switch causes feedthrough.

C. Uses

This circuit is used in Analog to Digital and Digital to Analog Converters.