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參數資料
| 型號： | AD7840ARS |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | DAC |
| 英文描述： | LC2MOS Complete 14-Bit DAC |
| 中文描述： | SERIAL, PARALLEL, WORD INPUT LOADING, 2.5 us SETTLING TIME, 14-BIT DAC, PDSO24 |
| 封裝： | SSOP-24 |
| 文件頁數： | 8/16頁 |
| 文件大小： | 337K |
| 代理商： | AD7840ARS |
AD7840
REV. B
–8–
As in the parallel mode, the
LDAC
signal controls the loading
of data to the DAC latch. Normally, data is loaded to the DAC
latch on the falling edge of
LDAC
. However, if
LDAC
is held
low, then serial data is loaded to the DAC latch on the sixteenth
falling edge of SCLK . If
LDAC
goes low during the transfer of
serial data to the input latch, no DAC latch update takes place
on the falling edge of
LDAC
. If
LDAC
stays low until the serial
transfer is completed, then the update takes place on the six-
teenth falling edge of SCLK . If
LDAC
returns high before the
serial data transfer is completed, no DAC latch update takes
place. Figure 9 shows the simplified serial input control logic for
the AD7840.
Figure 9. AD7840 Simplified Serial Input Control Logic
AD7840 DY NAMIC SPE CIFICAT IONS
T he AD7840 is specified and 100% tested for dynamic perfor-
mance specifications as well as traditional dc specifications such
as integral and differential nonlinearity. T hese ac specifications
are required for the signal processing applications such as
speech synthesis, servo control and high speed modems. T hese
applications require information on the DAC’s effect on the
spectral content of the signal it is creating. Hence, the param-
eters for which the AD7840 is specified include signal-to-noise
ratio, harmonic distortion and peak harmonics. T hese terms are
discussed in more detail in the following sections.
Signal-to-Noise Ratio (SNR)
SNR is the measured signal-to-noise ratio at the output of the
DAC. T he signal is the rms magnitude of the fundamental.
Noise is the rms sum of all the nonfundamental signals up to
half the sampling frequency (fs/2) excluding dc. SNR is depen-
dent upon the number of quantization levels used in the digiti-
zation process; the more levels, the smaller the quantization
noise. T he theoretical signal to noise ratio for a sine wave out-
put is given by
SNR =
(6.02
N
+ 1.76)
dB
(1)
where N is the number of bits. T hus for an ideal 14-bit con-
verter, SNR = 86 dB.
Figure 10 shows a typical 2048 point Fast Fourier T ransform
(FFT ) plot of the AD7840K N with an output frequency of
1 kHz and an update rate of 100 kHz. T he SNR obtained from
this graph is 81.8 dB. It should be noted that the harmonics are
taken into account when calculating the SNR.
Figure 10. AD7840 FFT Plot
E ffective Number of Bits
T he formula given in (1) relates the SNR to the number of bits.
Rewriting the formula, as in (2) it is possible to get a measure of
performance expressed in effective number of bits (N).
N
=
SNR
1.76
T he effective number of bits for a device can be calculated
directly from its measured SNR.
6.02
(2)
Harmonic Distortion
Harmonic distortion is the ratio of the rms sum of harmonics to
the fundamental. For the AD7840, total harmonic distortion
(T HD) is defined as
THD
=
20log
V
2
2
+
V
3
2
+
V
4
2
+
V
5
2
+
V
6
2
V
1
where V
1
is the rms amplitude of the fundamental and V
2
, V
3
,
V
4
, V
5
and V
6
are the rms amplitudes of the second through the
sixth harmonic. T he T HD is also derived from the 2048-point
FFT plot.
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 DAC output
spectrum (up to fs/2 and excluding dc) to the rms value of the
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.
T esting the AD7840
A simplified diagram of the method used to test the dynamic
performance specifications is outlined in Figure 11. Data is
loaded to the AD7840 under control of the microcontroller and
associated logic at a 100 kHz update rate. T he output of the
AD7840 is applied to a ninth order, 50 kHz, low-pass filter. T he
output of the filter is in turn applied to a 16-bit accurate digi-
tizer. T his digitizes the signal and the microcontroller generates
an FFT plot from which the dynamic performance of the
AD7840 can be evaluated.
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| AD7840BQ | 制造商:AD 制造商全稱:Analog Devices 功能描述:LC2MOS Complete 14-Bit DAC |
| AD7840CQ | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:14-Bit Digital-to-Analog Converter |
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