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| 型號： | AB-123 |
| 英文描述： | AB-123 - USING THE CONTINUOUS PARALLEL MODE WITH THE ADS7824 AND ADS7825 |
| 中文描述： | 抗體- 123 -利用連續(xù)平行模式，在ADS7824與ADS7825 |
| 文件頁數(shù)： | 1/6頁 |
| 文件大小： | 106K |
| 代理商： | AB-123 |
FIGURE 1. When converting rms noise to peak-to-peak
noise a crest factor can be used as a multiplying constant
which predicts the probability of peaks occurring beyond the
peak-to-peak noise calculation.
1
3
4
1997 Burr-Brown Corporation
AB-120
Printed in U.S.A. September, 1997
The ADS1210 and ADS1211 are precision, wide dynamic
range,
Σ
A/D converters that have 24 bits of no missing
code and up to 23 bits
rms of effective resolution. Because
of the nature of the
Σ
design architecture the circuit
designer has control over more of the variables and interface
options in the A/D’s conversion process than is realizable
with other converter architectures. These options are digi-
tally implemented and include analog input configuration,
oversampling, calibration and digital protocol control. Con-
sequently, these A/D converters can service a wide variety of
applications, including high precision, high speed, and low
power.
There is the subset of conditions that must exist before
23 bits
rms of resolution can be achieved with the
ADS1210/11. The manipulation of these A/D functions as
well as practical layout recommendations are discussed in
this application note.
23-BITS
rms
OF EFFECTIVE RESOLUTION DEFINED
Effective number of bits or effective resolution is a term that
was developed with the arrival of the 16+ bit converters.
These high resolution converters were capable of outputting
more bits than could accurately be digitized with one con-
version. With careful layout practices, this degree of uncer-
tainty was and still is predominately a consequence of
device noise. Multiple conversions, along with mathemati-
cal manipulation, reliably produces a higher effective reso-
lution at the expense of overall conversion speed. The
addition of a DSP or
μ
C type device is required in the
application to accomplish this type of performance improve-
ment. The topology of
Σ
converter relieves the board level
designer of the intensive DSP software design work by
incorporating the over sampling and digital filtering inside
the A/D chip.
The high resolution
Σ
converters, such as the ADS121x
family from Burr-Brown, advertise an effective resolution
up to 23 bits
rms in a 10Vp-p Full-Scale-Range at a 100Hz
data rate. This translates to 0.975
μ
Vrms of effective resolu-
tion. For the remainder of this discussion, “effective”
defines the rms digital output of the ADS1210/11 when
configured in a full-scale input range of 10V and a Program-
mable Gain setting of one. This performance exceeds other
A/D converter topologies, such as the SAR (Successive
Approximation) designs. As an extra bonus, the
Σ
con-
verter applications are less expensive than the precision
SAR converter applications.
Although the
Σ
converter absorbs the computational
overhead of the digital filtering function, there is a slight
variation for digital output to digital output. The accuracy of
the digital output code is affected by the cumulative noise at
the time of the conversion. This noise can be generated by
the circuit and injected into the A/D converter through the
input pins, reference pin or power supply connections.
Alternatively, the noise can also be generated by the device
itself. Effective resolution is defined as the statistical stan-
dard deviation (Vrms) of multiple conversions. A sample
size of 256 was used to characterize the ADS121x family of
products. Smaller sample sizes are also appropriate.
Although noise is a random event and any amplitude is
theoretically possible, the occurrence of each output over time
can be reliably predicted with the Gaussian distribution statis-
tical model. When the rms value is multiplied by twice the
crest factor, a peak-to-peak equivalent can be computed. The
Gaussian distribution, shown in Figure 1, illustrates that the
likelihood of large values decrease with increased magnitude.
The probability of exceeding a value above the rms (one
standard deviation) can be anticipated with a crest factor
(peak/rms). Peak-to-peak values can be predicted with a 2x
crest factor. Figure 1 illustrates the probability of a specific
output deviation for the average output vs the 2x crest factor
multiple. For instance, with an effective resolution of 1
μ
Vrms,
the probability of a sample exceeding
±
2.625
μ
V (2x crest
factor = 5.25) from the average output is 0.01. If a 2x crest
factor of 6.6 is applied, the probability of the output exceeding
±
3.3
μ
V of the average output is 0.001.
HOW TO GET 23 BITS OF EFFECTIVE RESOLUTION
FROM YOUR 24-BIT CONVERTER
By Bonnie C. Baker
1.0
0.1
0.01
0.001
0.0001
0.00001
0.000001
0.0000001
0.00000001
0.000000001
P-P NOISE CALCULATIONS FROM RMS
P
2
3
4
5
6
7
8
9
10
11
2x Crest Factor (Vp-p/Vrms)
12
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