AD15700

ADC DEFINITION OF SPECIFICATIONS

Integral Nonlinearity Error (INL)

Linearity error refers to the deviation of each individual code

from a line drawn from “negative full scale” through “positive

full scale.” The point used as negative full scale occurs 1/2 LSB

before the first code transition. Positive full scale is defined as a

level 1 1/2 LSB beyond the last code transition. The deviation is

measured from the middle of each code to the true straight line.

Differential Nonlinearity Error (DNL)

In an ideal ADC, code transitions are 1 LSB apart. Differential

nonlinearity is the maximum deviation from this ideal value.

It is often specified in terms of resolution for which no missing

codes are guaranteed.

Full-Scale Error

The last transition (from 011...10 to 011...11 in twos comple-

ment coding) should occur for an analog voltage 1 1/2 LSB

below the nominal full scale (2.499886 V for the ± 2.5 V range).

The full-scale error is the deviation of the actual level of the last

transition from the ideal level.

Bipolar Zero Error

The difference between the ideal midscale input voltage (0 V)

and the actual voltage producing the midscale output code.

Unipolar Zero Error

In unipolar mode, the first transition should occur at a level

1/2 LSB above analog ground. The unipolar zero error is the

deviation of the actual transition from that point.

Spurious Free Dynamic Range (SFDR)

The difference, in decibels (dB), between the rms amplitude of

the input signal and the peak spurious signal.

Effective Number of Bits (ENOB)

A measurement of the resolution with a sine wave input. It is

related to S/(N + D) by the following formula:

(( ) ) [ ] ENOB = S / N + D dB – 1.76 / 6.02

and is expressed in bits.

Total Harmonic Distortion (THD)

The rms sum of the first five harmonic components to the rms

value of a full-scale input signal; expressed in decibels.

Signal-to-Noise Ratio (SNR)

The ratio of the rms value of the actual input signal to the rms

sum of all other spectral components below the Nyquist frequency,

excluding harmonics and dc. The value for SNR is expressed in

decibels.

Signal-to-(Noise + Distortion)

Ratio (S/[N + D])

The ratio of the rms value of the actual input signal to the rms

sum of all other spectral components below the Nyquist frequency,

including harmonics but excluding dc. The value for S/(N + D)

is expressed in decibels.

Aperture Delay

A measure of the acquisition performance, measured from the

falling edge of the CNVST input to when the input signal is

held for a conversion.

Transient Response

The time required for the ADC to achieve its rated accuracy

after a full-scale step function is applied to its input.

DAC DEFINITION OF SPECIFICATIONS

Relative Accuracy

For the DAC, relative accuracy or integral nonlinearity (INL)

is a measure of the maximum deviation in LSBs from a straight

line passing through the endpoints of the DAC transfer function.

A typical INL versus code plot can be seen in TPC 16.

Differential Nonlinearity

Differential nonlinearity is the difference between the measured

change and the ideal 1 LSB change between any two adjacent

codes. A specified differential nonlinearity of ± 1 LSB maximum

ensures monotonicity. TPC 19 illustrates a typical DNL versus

code plot.

Gain Error

Gain error is the difference between the actual and ideal analog

output range, expressed as a percent of the full-scale range. It is

the deviation in slope of the DAC transfer characteristic from ideal.

Gain Error Temperature Coefficient

This is a measure of the change in gain error with changes in

temperature. It is expressed in ppm/∞C.

Zero Code Error

Zero code error is a measure of the output error when zero code

is loaded to the DAC register.

Zero Code Temperature Coefficient

This is a measure of the change in zero code error with a change

in temperature. It is expressed in mV/∞C.

Digital-to-Analog Glitch Impulse

Digital-to-analog glitch impulse is the impulse injected into the

analog output when the input code in the DAC register changes

state. It is normally specified as the area of the glitch in nV–s

and is measured when the digital input code is changed by 1 LSB

at the major carry transition. A plot of the glitch impulse is shown

in Figure 28.

Digital Feedthrough

Digital feedthrough is a measure of the impulse injected into the

analog output of the DAC from the digital inputs of the DAC, but

is measured when the DAC output is not updated. CS_DAC is

held high, while the CLK and DIN signals are toggled. It is

specified in nV–s and is measured with a full-scale code change

on the data bus, i.e., from all 0s to all 1s and vice versa. A typical

plot of digital feedthrough is shown in Figure 27.

Power Supply Rejection Ratio

This specification indicates how the output of the DAC is affected

by changes in the power supply voltage. Power supply rejection

ratio is quoted in terms of percent change in output per percent

change in VDD for full-scale output of the DAC. VDD is varied

by ± 10%.

Reference Feedthrough

This is a measure of the feedthrough from the VREF input to the

DAC output when the DAC is loaded with all 0s. A 100 kHz,

1 V p-p is applied to VREF. Reference feedthrough is expressed

in mV p-p.

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REV. A