AD7893
The same data is presented in Figure 11 as in Figure 10 except
that, in this case, the output data read for the device occurs dur-
ing conversion. This has the effect of injecting noise onto the die
while bit decisions are being made; this increases the noise gen-
erated by the AD7893. The histogram plot for 8192 conversions
of the same dc input now shows a larger spread of codes with
the rms noise for the AD7893-2 increasing to 210 µV. This ef-
fect will vary depending on where the serial clock edges appear
with respect to the bit trials of the conversion process. It is pos-
sible to achieve the same level of performance when reading
during conversion as when reading after conversion, depending
on the relationship of the serial dock edges to the bit trial points.
Effective Number of Bits
The formula for signal to (noise + distortion) ratio (see Termi-
nology section) is related to the resolution or number of bits in
the converter. Rewriting the formula gives a measure of perfor-
mance expressed in effective number of bits (N):
N = (SNR – 1.76) / 6.02
where SNR is Signal to (Noise + Distortion) Ratio.
The effective number of bits for a device can be calculated from
its measured signal to (noise + distortion) ratio. Figure 13 shows
a typical plot of effective number of bits versus frequency for the
AD7893-2 from dc to fSAMPLING/2. The sampling frequency is
102.4 kHz. The plot shows that the AD7893 converts an input
sine wave of 51.2 kHz to an effective numbers of bits of 11,
which equates to a signal to (noise + distortion) level of 68 dB.
7500
7000
SAMPLING
FREQUENCY = 102.4kHz
6500
6000
5500
5000
4500
4000
3500
3000
2500
2000
1500
T
= +25°C
A
12.0
11.5
11.0
10.5
10.0
1000
500
0
(X–4) (X–3) (X–2) (X–1)
X
(X+1) (X+2) (X+3) (X+4)
CODE
Figure 11. Histogram of 8192 Conversions with Read Dur-
ing Conversion
0
25.6
51.2
INPUT FREQUENCY – kHz
Dynamic Performance
With a combined conversion and acquisition time of 7.5 µs, the
AD7893 is ideal for wide bandwidth signal processing applica-
tions. These applications require information on the ADC’s
effect on the spectral content of the input signal. Signal to (noise
+ distortion) ratio, total harmonic distortion, peak harmonic or
spurious noise, and intermodulation distortion are all specified.
Figure 12 shows a typical FFT plot of a 10 kHz, 0 V to +2.5 V
input after being digitized by the AD7893-2, operating at a
102.4 kHz sampling rate. The signal to (noise + distortion)
ratio is 71.5 dB, and the total harmonic distortion is –83 dB.
Figure 13. Effective Number of Bits vs. Frequency
0
SAMPLE RATE = 102.4kHz
INPUT FREQUENCY = 10kHz
SNR = 71.5dB
T
= +25°C
–30
–60
A
–80
–120
–180
0
25.6
51.2
FREQUENCY – kHz
SNR IS SIGNAL TO (NOISE AND DISTORTION) RATIO
Figure 12. AD7893 FFT Plot
REV. E
–11–
ADI [ ADI ]
