AD7862

–12–

REV. 0

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 . . ., etc. Intermodulation terms are those for

which neither m or n are equal to zero. For example, the second

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

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

Using the CCIF standard where two input frequencies near the

top end of the input bandwidth are used, the second and third

order terms are of different significance. 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 inter-

modulation 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 fundamental expressed in dBs. In this

case the input consists of two, equal amplitude, low distortion

sine waves. Figure 11 shows a typical IMD plot for the AD7862.

–0

–120

0

12.3k

10k

30k

50k

70k

90k

–20

–40

–60

–80

–100

–10

–30

–50

–70

–90

–110

100k

INPUT FREQUENCIES

F1 = 50010 Hz

F2 = 49110 Hz

SNR = –60.62dB

THD = –89.22dB

IMD:

2ND ORDER TERM –88.44 dB

3RD ORDER TERM –66.20 dB

Figure 11. AD7862 IMD Plot

Peak Harmonic or Spurious Noise

Harmonic or spurious noise is defined as the ratio of the rms

value of the next largest component in the ADC output spec-

fundamental. Normally, the value of this specification will be

determined by the largest harmonic in the spectrum, but for

parts where the harmonics are buried in the noise floor, the peak

will be a noise peak.

AC Linearity Plot

input of the AD7862, 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 12. This shows very good integral linearity performance

from the AD7862 at an input frequency of 10 kHz. The absence

of large spikes in the plot shows good differential linearity. Sim-

plified versions of the formulas used are outlined below.

INL(i )

=

Vi

()

















− i

are the estimated full-scale and offset transitions, and V(i) is the

estimated transition for the i

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

V (i )

=− A × Cos

π× cum i

()

N

















where A is the peak signal amplitude, N is the number of

histogram samples

and cum i

n

=0

i

∑

occurrences

0

–0.1

–0.2

–0.3

–0.4

–0.5

0.5

0.4

0.3

0.2

0.1

LSB

Figure 12. AD7862 AC INL Plot

Power Considerations

In the automatic power-down mode the part may be operated at

a sample rate that is considerably less than 200 kHz. In this

case, the power consumption will be reduced and will depend

on the sample rate. Figure 13 shows a graph of the power

consumption versus sampling rates from 100 Hz to 90 kHz in

the automatic power-down mode. The conditions are 5 V

supply 25

°C, and the data was read after conversion.

0.1

90

10

20

30

40

0

25

20

15

10

35

30

40

5

FREQUENCY – kHz

50

60

70

80

Figure 13. Power vs. Sample Rate in Auto Power-Down

Mode