AD7894 Analog Devices, AD7894 Datasheet - Page 5

• Manufacturer Part Number: AD7894
• Manufacturer: Analog Devices

Specifications of AD7894

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 200kSPS
• Interface: Ser
• Analog Input Type: SE-Bip,SE-Uni
• Ain Range: Bip (Vref),Bip (Vref) x 4,Bip 10V,Bip 2.5V,Uni (Vref),Uni 2.5V
• Adc Architecture: SAR
• Pkg Type: 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 quan-
tization noise. The theoretical signal to (noise + distortion) ratio
for an ideal N-bit converter with a sine wave input is given by:
Thus for a 14-bit converter, this is 86.04 dB.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7894, it is defined as:
where V
V
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 f
fundamental. The value of this specification is normally 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 is 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).
The AD7894 is tested using two input frequencies. In this case,
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 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.
4
, V
5
and V
Signal to (Noise + Distortion) = (6.02 N + 1.76) dB
1
THD dB
is the rms amplitude of the fundamental and V
6
are the rms amplitudes of the second through the
(
S
/2 and excluding dc) to the rms value of the
)
20
log
V
2
2
V
3
2
V
V
1
4
S
2
/2), excluding dc.
V
5
2
V
nfb where
6
2
2
, V
3
,
–5–
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.
Positive Gain Error (AD7894-10)
This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal (4 VREF – 1 LSB) after the
Bipolar Zero Error has been adjusted out.
Positive Gain Error (AD7894-3)
This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal (VREF – 1 LSB) after the Bipolar
Zero Error has been adjusted out.
Positive Gain Error (AD7894-2)
This is the deviation of the last code transition (11 . . . 110 to
11 . . . 111) from the ideal (VREF – 1 LSB) after the Unipolar
Offset Error has been adjusted out.
Bipolar Zero Error (AD7894-10, AD7894-3)
This is the deviation of the midscale transition (all 0s to all 1s)
from the ideal 0 V (GND).
Unipolar Offset Error (AD7894-2)
This is the deviation of the first code transition (00 . . . 000 to
00 . . . 001) from the ideal 1 LSB.
Negative Gain Error (AD7894-10)
This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal (–4 VREF + 1 LSB) after Bipolar
Zero Error has been adjusted out.
Negative Gain Error (AD7894-3)
This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal (– VREF + 1 LSB) after Bipolar
Zero 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/hold returns to track mode). It also applies to situations
where there is a step input change on the input voltage applied
to the V
wait for the duration of the track/hold acquisition time after the
end of conversion or after a step input change to V
starting another conversion, to ensure that the part operates to
specification.
1/2 LSB, after the end of conversion (the point at which the
IN
input of the AD7894. This means that the user must
AD7894
IN
before