AD7827 Analog Devices, AD7827 Datasheet - Page 5

• Manufacturer Part Number: AD7827
• Description: 3/5V, 1 MSPS, 8-Bit, Serial Interface Sampling ADC
• Manufacturer: Analog Devices

Specifications of AD7827

• Resolution (bits): 8bit
• # Chan: 1
• Sample Rate: 1MSPS
• Interface: Ser
• Analog Input Type: SE-Uni
• Ain Range: Uni 2.0V,Uni 2.5V
• Adc Architecture: Pipelined
• Pkg Type: DIP,SOIC

REV. 0
TERMINOLOGY
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 rms sum of all nonfundamental
signals up to half the sampling frequency (f
The ratio is dependent upon the number of quantization levels in
the digitization process; the more levels, the smaller the quantiza-
tion noise. The theoretical signal-to-(noise + distortion) ratio for
an ideal N-bit converter with a sine wave input is given by:
Thus for an 8-bit converter, this is 50 dB.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7827 it is defined as:
where V
V
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 f
fundamental. Normally, the value of this specification is deter-
mined by the largest harmonic in the spectrum, but for parts
where the harmonics 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
m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for
which neither m nor n are equal to zero. For example, the sec-
ond order terms include (fa + fb) and (fa – fb), while the third
order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb).
The AD7827 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 and third order terms are of dif-
ferent significance. The second order terms are usually dis-
tanced in frequency from the original sine waves while the third
5
and V
Signal-to-(Noise + Distortion) = (6.02N + 1.76) dB
THD (dB) 20 log
1
6
is the rms amplitude of the fundamental and V
are the rms amplitudes of the second through the sixth
S
/2 and excluding dc) to the rms value of the
V
2
2
V
3
2
V
V
S
1
/2), excluding dc.
4
2
V
5
2
V
nfb where
6
2
2
, V
3
, V
4
,
–5–
order terms are usually at a frequency close to the input fre-
quencies. 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 fundamental expressed in dBs.
Relative Accuracy
Relative accuracy or endpoint nonlinearity is the maximum
deviation from a straight line passing through the endpoints of
the ADC transfer function.
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 128th code transition (01111111) to
(10000000) from the ideal, i.e., V
(V
Zero Scale Error
This is the deviation of the first code transition (00000000) to
(00000001) from the ideal, i.e., V
5 V
Full-Scale Error
This is the deviation of the last code transition (11111110) to
(11111111) from the ideal, i.e., V
5 V
Gain Error
This is the deviation of the last code transition (1111 . . . 110) to
(1111 . . . 111) from the ideal, i.e., V
error has been adjusted out.
Track/Hold Acquisition Time
Track/hold acquisition time is the time required for the output
of the track/hold amplifier to reach its final value, within
track mode. This happens approximately 120 ns after the falling
edge of CONVST.
It also applies when there is a step input change on the input
voltage applied to the V
the user must wait for the duration of the track/hold acquisition
time after the end of conversion or after a step input change to
V
operates to specification.
1/2 LSB, after the point at which the track/hold returns to
IN
DD
before starting another conversion, to ensure that the part
= 3 V).
10%), or 0.8 V
10%), or V
MID
+ 1.0 V – 1 LSB (V
REF
IN
/2 –1.0 V + 1 LSB (V
input of the AD7827. It means that
REF
REF
MID
/2 –1.25 V + 1 LSB (V
/2 (V
REF
+ 1.25 V – 1 LSB (V
– 1 LSB, after the offset
DD
DD
= 3 V
= 5 V), 0.8 V
DD
AD7827
= 3 V
10%).
10%).
DD
REF
DD
=
/2
=