AD7490BCP datasheet(7/24 Pages) AD

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AD7490

–7–

REV. A

TERMINOLOGY

Integral Nonlinearity

This is the maximum deviation from a straight line passing through

the endpoints of the ADC transfer function. The endpoints of

the transfer function are zero scale, a point 1 LSB below the first

code transition, and full scale, a point 1 LSB above the last code

transition.

Differential Nonlinearity

This is the difference between the measured and the ideal 1 LSB

change between any two adjacent codes in the ADC.

Offset Error

This is the deviation of the first code transition (00…000) to

(00…001) from the ideal, i.e., AGND

1 LSB.

Offset Error Match

This is the difference in offset error between any two channels.

Gain Error

This is the deviation of the last code transition (111…110) to

(111…111) from the ideal (i.e., REFIN

1LSB) after the offset

error has been adjusted out.

Gain Error Match

This is the difference in gain error between any two channels.

Zero Code Error

This applies when using the twos complement output coding

option, in particular to the 2

REFIN input range with –REFIN

to +REFIN biased about the REFIN point. It is the deviation of the

midscale transition (all 0s to all 1s) from the ideal VIN voltage,

i.e., REFIN – 1 LSB.

Zero Code Error Match

This is the difference in zero code error between any two channels.

Positive Gain Error

This applies when using the twos complement output coding

option, in particular to the 2

REFIN input range with –REFIN

to +REFIN biased about the REFIN point. It is the deviation of

the last code transition (011…110) to (011…111) from the ideal

(i.e., +REFIN

1 LSB) after the Zero Code Error has been

adjusted out.

Positive Gain Error Match

This is the difference in Positive Gain Error between any two

channels.

Negative Gain Error

This applies when using the twos complement output coding

option, in particular to the 2

REFIN input range with –REFIN

to +REFIN biased about the REFIN point. It is the deviation of

the first code transition (100…000) to (100…001) from the

ideal (i.e., –REFIN + 1 LSB) after the Zero Code Error has been

adjusted out.

Negative Gain Error Match

This is the difference in negative gain error between any two

channels.

Channel-to-Channel Isolation

Channel-to-channel isolation is a measure of the level of cross-

talk between channels. It is measured by applying a full-scale

400 kHz sine wave signal to all 15 nonselected input channels and

determining how much that signal is attenuated in the selected

channel with a 50 kHz signal. The figure is given worst case

across all 16 channels for the AD7490.

PSR (Power Supply Rejection)

Variations in power supply will affect the full scale transition, but not

the converter’s linearity. Power supply rejection is the maximum

change in full-scale transition point due to a change in power-

supply voltage from the nominal value. (See Typical Performance

Characteristics.)

Track/Hold Acquisition Time

The track/hold amplifier returns into track on the 14th SCLK

falling edge. Track/hold acquisition time is the minimum time

required for the track and hold amplifier to remain in track

mode for its output to reach and settle to within

±1 LSB of the

applied input signal, given a step change to the input signal.

Signal to (Noise + Distortion) Ratio

This is the measured ratio of signal to (noise + distortion) at the

output of the A/D converter. The signal is the rms amplitude of

the fundamental. Noise is the sum of all nonfundamental signals

up to half the sampling frequency (fS/2), excluding dc. The ratio

is dependent on the number of quantization levels in the digitiza-

tion process; the more levels, the smaller the quantization noise.

The theoretical signal to (noise + distortion) ratio for an ideal

N-bit converter with a sine wave input is given by:

Signal to Noise

Distortion

() ( .

)

602

176

Thus for a 12-bit converter, this is 74 dB.

Total Harmonic Distortion

Total harmonic distortion (THD) is the ratio of the rms sum of

harmonics to the fundamental. For the AD7490, it is defined as:

THD dB

VVV

()

log

=×

where V1 is the rms amplitude of the fundamental and V2, V3,

V4, V5, and V6 are the rms amplitudes of the second through the

sixth harmonics.

Peak Harmonic or Spurious Noise

Peak harmonic or spurious noise is defined as the ratio of the rms

value of the next largest component in the ADC output spectrum

(up to fS/2 and excluding dc) to the rms value of the fundamental.

Normally, the value of this specification is determined by the

largest harmonic in the spectrum, but for ADCs where the har-

monics are buried in the noise floor, it will be a noise peak.

Intermodulation Distortion

With inputs consisting of sine waves at two frequencies, fa and fb,

any active device with nonlinearities will create distortion products

at sum and difference frequencies of mfa

± nfb, where m, n = 0,

1, 2, 3, and so on. Intermodulation distortion terms are those for

which neither m nor n are equal to zero. For example, the second

order terms include (fa + fb) and (fa – fb), while the third order

terms include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb).

The AD7490 is tested using the CCIF standard where two input

frequencies near the top end of the input bandwidth are used.

In this case, the second order terms are usually distanced in

frequency from the original sine waves while the third order terms

are usually at a frequency close to the input frequencies. As a

result, the second and third order terms are specified separately.

The calculation of the intermodulation distortion is as per the

THD specification, where it is the ratio of the rms sum of the

individual distortion products to the rms amplitude of the sum

of the fundamentals expressed in dBs.

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