AD7870LP datasheet(18/28 Pages) AD

18 / 28 page

AD7870/AD7875/AD7876

Rev. C | Page 18 of

–30

–60

–90

–120

V(i), the estimated code transition point, is derived as follows:

FREQUENCY (kHz)

INPUT FREQUENCIES

F1 = 9.05kHz

F2 = 9.55kHz

SAMPLING FREQUENCY = 100kHz

TA = 25°C

IMD

ALL TERMS = 90.06dB

SECOND ORDER TERMS = 92.73dB

THIRD ORDER TERSM = 93.45dB

()

[

]

cum

Cos

−

where:

A is the peak signal amplitude.

N is the number of histogram samples.

cum(i) =

()

∑

occurrences.

0.5

–0.50

0.25

–0.25

511

1023

1535

2047

2559

3071

3583

4095

CODE

INPUT FREQUENCY = 25kHz

SAMPLE FREQUENCY = 100kHz

TA = 25°C

Figure 20. IMD Plot

AC Linearity Plot

When a sine wave of specified frequency is applied to the VIN

input of the AD7870/AD7875 and several million samples are

taken, a histogram showing the frequency of occurrence of each

of the 4096 ADC codes can be generated. From this histogram

data it is possible to generate an ac integral linearity plot as

shown in Figure 21. This shows very good integral linearity

performance from the AD7870/AD7875 at an input frequency

of 25 kHz. The absence of large spikes in the plot shows good

differential linearity. Simplified versions of the formulae used

are outlined below.

Figure 21. AC INL Plot

()

() ()

INL

−

⎥

⎦

⎤

⎢

⎣

⎡

−

4096

where:

INL(i) is the integral linearity at code i.

V(fs) and V(o) are the estimated full-scale and offset transitions.

V(i) is the estimated transition for the ith code.

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