REV. A

AD7475/AD7495

–7–

PIN FUNCTION DESCRIPTIONS

Pin

No.

Mnemonic

Function

1

REF IN

Reference Input for the AD7475. An external reference must be applied to this input. The voltage range

for the external reference is 2.5 V

±1% for specified performance. A cap of a least 0.1 F should be placed

on the REF IN pin.

REF OUT

Reference Output for the AD7495. A minimum 100 nF capacitance is required from this pin to GND. The

internal reference can be taken from this pin but buffering is required before it is applied elsewhere in a system.

Analog Input. Single-ended analog input channel. The input range is 0 to REF IN.

3

GND

Analog Ground. Ground reference point for all circuitry on the AD7475/AD7495. All analog input signals

and any external reference signal should be referred to this GND voltage.

4

SCLK

Serial Clock. Logic input. SCLK provides the serial clock for accessing data from the part. This clock input is

also used as the clock source for the AD7475/AD7495’s conversion process.

5

SDATA

Data Out. Logic Output. The conversion result from the AD7475/AD7495 is provided on this output as a

serial data stream. The bits are clocked out on the falling edge of the SCLK input. The data stream consists

of four leading zeros followed by the 12 bits of conversion data which is provided MSB first.

Logic Power Supply Input. The voltage supplied at this pin determines at what voltage the serial interface

of the AD7475/AD7495 will operate.

7

CS

Chip Select. Active low logic input. This input provides the dual function of initiating conversions on the

AD7475/AD7495 and also frames the serial data transfer.

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 1/2 LSB below the first

code transition, and full scale, a point 1/2 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 + 0.5 LSB.

Gain Error

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

error has been adjusted out.

Track/Hold Acquisition Time

The track/hold amplifier returns into track mode on the 13th

SCLK rising edge (see Serial Interface section). The 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 0.5 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

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

zation 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) = (6.02 N + 1.76) dB

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 AD7475/AD7495, it is

defined as:

THD dB

VVVVV

V

(

)

log

=

++++

20

2

2

3

2

4

2

5

2

6

2

1

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

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, etc. Intermodulation distortion terms are those

for which neither m nor n is 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 AD7475/AD7495 are 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 sepa-

rately. 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.