AD1876JN Analog Devices, AD1876JN Datasheet - Page 9

• Manufacturer Part Number: AD1876JN
• Description: 16-Bit 100 kSPS Sampling ADC
• Manufacturer: Analog Devices

REV. A
corresponding pin. The capacitor is disconnected when SAMPLE
is taken LOW and the stored charge is used in the subsequent
A/D conversion. In order to limit the demands placed on the
external source by this high initial charging current, an internal
buffer amplifier is employed between the input and this capaci-
tance for a few hundred nanoseconds. During this time the
input pin exhibits typically 20 k input resistance, 10 pF input
capacitance and 40 A bias current. Next, the input is switched
directly to the now precharged capacitor and allowed to fully
settle, after which SAMPLE is taken LOW. During this time
the input sees only a 50 pF capacitor. Once the sample is taken,
the input is internally floated so that the external input source
sees a very high input resistance and a parasitic input capaci-
tance of typically only 2 pF. As a result, the only dominant input
characteristic which must be considered is the high current steps
which occur when the internal buffers are switched in and out.
In most cases, it is desirable to use external op amps to drive the
AD1876. For ac applications where low cost and low distortion
are desired, the AD711 may be used as shown in Figure 7. An-
other option is the 5532/5534 series. Care should always be
taken with op amp selection—many available op amps do not
meet the necessary low distortion requirements with even mod-
erate loading conditions.
TESTING THE AD1876
Analog Devices employs a high performance mixed signal VLSI
tester to verify the electrical performance of every AD1876. The
test system consists of two main sections, an input signal gen-
erator and a digital data and control section.
The stimulus section is responsible for providing a high purity,
noise-free, band limited tone to the input of the device. This in-
put frequency is 1.06 kHz. The test tone is passed through a
bandpass filter to remove distortion products and then buffered
by a high performance op amp. An external 5.000 V reference
voltage is also supplied by this section.
The control section of the test equipment provides an external
clock and the control signals for calibration, conversion and data
transmission. This section of the tester also contains the pro-
cessing unit that calculates the actual performance of the device
under test.
V
IN
1k
499
2
3
AD711
Figure 7.
1k
–12V
+12V
7
4
0.1µF
0.1µF
6
10
8
9
V
AGND
AGND SENSE
IN
AD1876
–9–
The test procedure consists of the following steps. First, the
device is calibrated by its on-board controller. Next, the device
under test digitizes the input waveform. This conversion is
performed at a 96 kSPS rate and transmits the resulting serial
data to the tester. The tester performs an FFT on the test data
and determines the actual performance of the device.
AC PERFORMANCE
Using the aforementioned test methodology, ac performance
of the AD1876 is measured. AC parameters, which include
S/(N+D), THD, etc., reflect the AD1876’s effect on the spec-
tral content of the analog input signal. Figures 11 through 15
provide information on the AD1876’s ac performance under a
variety of conditions.
As a general rule, averaging the results from several conversions
reduces the effects of noise and, therefore, improves such pa-
rameters as S/(N+D) and THD. AD1876 performance is opti-
mized by operating the device at its maximum sample rate of
100 kSPS and digitally filtering the resulting bit stream to the
desired signal bandwidth. This succeeds in distributing noise
over a wider frequency range, thus reducing the noise density in
the frequency band of interest. This subject is discussed in the
following section.
OVERSAMPLING AND NOISE FILTERING
The Nyquist rate for a converter is defined as one-half its sam-
pling rate. This is established by the Nyquist theorem, which
requires that a signal be sampled at a rate corresponding to at
least twice its widest bandwidth of interest in order to preserve
the information content. Oversampling is a conversion tech-
nique in which the sampling frequency is an integral (2 or more)
multiple of twice the frequency bandwidth of interest. In audio
applications, the AD1876 can operate at a 2 oversampling rate.
In quantized systems, the information content of the analog in-
put is represented in the frequency spectrum from dc to the
Nyquist rate of the converter. Within this same spectrum are
higher frequency aliased noise components. Antialias, or low-
pass, filters are used at the input to the ADC to remove the por-
tion of these noise components attributed to high frequency
analog input noise. However, wideband noise contributed by the
AD1876 will not be reduced by the antialias filter. The AD1876
contributed noise is evenly distributed from dc to the Nyquist
rate, and this fact can be used to minimize its overall effect.
The AD1876 contributed noise effects can be reduced by
oversampling—sampling at a rate higher than defined by the
Nyquist theorem. This spreads the noise energy over a distribu-
tion of frequencies wider than the frequency band of interest,
and by judicious selection of a digital filter, noise frequencies
outside the bandwidth of interest may be eliminated. The pro-
cess of quantization inherently produces noise, known as quanti-
zation noise. The magnitude of this noise is a function of the
resolution of the converter, and manifests itself as a limit to the
theoretical signal-to-noise ratio achievable. This limit is de-
scribed by S/(N+D) = (6.02 n + 1.76 + 10 log F
where n is the resolution of the converter in bits, F
pling frequency, and F
audio bandwidth applications, the AD1876 is capable of operat-
ing at a 2 oversample rate (96 kSPS), which typically produces
an improvement in S/(N+D) of 3 dB compared with operating
at the Nyquist conversion rate of 48 kSPS. Oversampling has
another advantage as well; the demands on the antialias filter are
a
is the signal bandwidth of interest. For
AD1876
S
/2 F
S
is the sam-
a
) dB,