Test Parameter

Unit

Typical Description

Resolution

Bits

If an ADC has n bits, then its resolution is 2ⁿ, which is the number of states or codes that it can divide (or "resolve") the analog input into. The higher the number of bits, the better is the resolution of the ADC, and the more precise the representation of each state will be. 

Unipolar Offset

LSB

The first transition of an ADC should occur at an analog value that is 1/2 LSB above the analog common.  The deviation of the ADC's actual first transition from this ideal level is known as the unipolar offset.

Example of an Actual Spec: +/- 1 LSB max

Bipolar Offset or

Bipolar Zero Error

LSB

In the bipolar mode, the major carry transition from '011111111111' to '100000000000' should occur at an analog input value that is 1/2 LSB below the analog common. The deviation of the actual value of this transition at 25 deg C from the ideal level is known as the Bipolar Offset or Bipolar Zero Error. 

Example of an Actual Spec: +/- 1 LSB max

Non-Linearity, Differential (DNL)

Bits

(with no missing codes)

In an ADC, code transitions are 1 LSB apart.  Maximum deviation from this ideal value is known as differential non-linearity. It is often expressed in terms of the number of bits guaranteed to have no missing codes.

Example of an Actual Spec: 10 Bits min

Non-Linearity, Integral (INL)

LSB

The transfer function of an ADC is a straight line from "zero" to "full scale".  The maximum deviation of a code from this straight line is the ADC's integral non-linearity.

Example of an Actual Spec: +/- 2 LSB's max

Analog Input Range or Full-Scale Range

V

This is the difference between the maximum and minimum analog input values specified for the ADC.

Examples of Actual Specs:

0V to +10 V, Unipolar Mode;

-5V to +5V, Bipolar Mode

Logic Output

(Current Sink)

mA

This is the amount of current that the digital output of an ADC can sink when it is at Logic "0".

Example of an Actual Spec:

3.2 mA when Vout = 0.4V max

Logic Output

(Current Source)

mA

This is the amount of current that the digital output of an ADC can source when it is at Logic "1".

Example of an Actual Spec:

0.5 mA when Vout = 2.4V max

Logic Input Voltage

(Logic "1")

V

This is the minimum voltage that the digital inputs of the ADC are guaranteed to recognize as a Logic "1".

 Example of an Actual Spec: 2.0 V min.

Logic Input Voltage

(Logic "0")

V

This is the maximum voltage that the digital inputs of the ADC are guaranteed to recognize as a Logic "0".

Example of an Actual Spec: 0.8 V max.

Conversion Time

µsec

This is the time required by an ADC to make a complete measurement.

Example of an Actual Spec:

15 µsec min.; 25 µsec typ.; 40 µsec max.

+ Power Supply (V+)

V

This is the voltage range that can be used for the positive supply of the ADC.

Example of an Actual Spec:

+4.5V min.; +5.0V typ.; +7.0V max.

- Power Supply (V-)

V

This is the voltage range that can be used for the negative supply of the ADC.

Example of an Actual Spec:

-12.0V min.; -15V typ.; -16.5V max.

Analog Input Impedance

kW

This is the dynamic load presented by an ADC to its analog input source. 

Example of an Actual Spec:

3 kW min.; 5 kW typ.; 7 kW max.

Gain Error

LSB

This is the deviation of the actual difference between the first and last transitions of an ADC from the ideal difference between its first and last transitions.  The last transition of the ADC should occur at an analog input value that is 1.5 LSB's below the nominal full scale.

Example of an Actual Spec:

+/- 2 LSB's typ.; +/-4 LSB's max.

Power Supply Rejection

LSB

This is the maximum change in the full-scale transition point of the ADC due to a deviation of its power supply voltage from its nominal value. This is measured at full-scale.

Examples of  Actual Specs:

+/- 1 LSB max for V+ = +4.5V to +5.5V;

+/- 1 LSB max for V- = -12.6V to -11.4V

Signal-to-Noise Ratio (SNR)

dB

This is the ratio of the measured signal at the output of the ADC to the measured noise. 'Signal' is the RMS magnitude of the fundamental signal while noise is the RMS sum of all non-fundamental signals up to half the sampling frequency, excluding the DC signal. 

Example of an Actual Spec:

44 dB typ. for Fin=1 MHz and Ain=+/-1V

Total Harmonic Distortion (THD)

% or dB

This is the ratio of the rms sum of the first six harmonic components to the rms value of a full-scale input signal, expressed either in % or dB. The aliased component is used for input signals or harmonics that are above the Nyquist frequency. The Nyquist frequency is the input signal frequency that is 1/2 the sampling frequency of the ADC.

Examples of Actual Specs:

-88 dB typ.; -80 dB max.

0.004% typ.; 0.010% max.