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| 型号： | AB-151 |
| 英文描述： | AB-151 - FOUR-WIRE RTD CURRENT-LOOP TRANSMITTER: Four-Wire Connections to an RTD Allow the RTD to be Remotely Located from Active Circuitry. Yet Maintain Accuracy |
| 中文描述： | 抗体- 151 -四线RTD电流环变送器：四线连接到一个RTD的电阻允许被远程位于从有源电路。然而，保持准确度 |
| 文件页数： | 4/5页 |
| 文件大小： | 78K |
| 代理商： | AB-151 |
4
noise, the spectral density of the voltage noise is propor-
tional to the value of resistance. The one on the right is the
Thevinen equivalent: a current source in parallel with a
noiseless resistor. Notice that the spectral density of the
current noise is inversely proportional to the value of resis-
tance. That is, the bigger the resistance, the smaller the
current noise. The DDC112 measures current and therefore,
the noise contribution of the resistor is best modeled using
the current source equivalent circuit. As seen in Figure 10,
the current noise is proportional to the inverse of the resistor’s
value. Notice that the thermal noise power of the resistor is
independent of its value. This comes from the physical
nature of thermal noise in a resistor and is explained in detail
in Reference 1.
As just described qualitatively, the resistor’s noise contribu-
tion seen at the DDC112’s output decreases as its value
increases. Now, to get some quantitative results to calculate
the actual amount of additional noise produced by the
resistor, equations are needed. In general, for a linear sys-
tem, the mean-squared output noise as a function of the
system’s transfer function and input noise is given by
where S() is the spectral noise density of the input noise and
H()
is the transfer function of the linear system. Sometimes,
the integral in Equation 2 is shown ranging from –
∞
to +
∞
but here, only positive frequencies are considered. The
transfer function can be found by taking the Fourier trans-
form of the impulse response of the system. For the DDC112,
it is the front-end integrators that set its overall transfer
function—the DDC112’s voltage-input A/D converter al-
ways samples the held value of the integrators and doesn’t
affect the overall frequency response. The integrator’s im-
pulse response is simply a pulse of width T
INT
. Working
“ppm” numbers directly from the plots. Figure 9 illustrates
a typical time plot using the DUT board setup of Figure 7:
T
INT
equals 500
μ
s, the DDC112’s Range is set at 250pC and
a 20M
resistor generates the offset current. The input
signal is zero. Reading the data from the upper right-hand
corner of the time plot, the average value (Yavg) is 0.4256905,
roughly a DDC112 output code of 450,464. The tolerances
in Q
FS
, R, and V cause the difference between Figure 9's
Yavg and the predicted value of 0.412 shown in Figure 8.
The rms noise (Yrms) is 0.0000038 out of full scale of 1.0
or 3.8 ppm of full scale.
THERMAL NOISE OF THE RESISTOR
At first glance, the very large resistor placed in series with
the input of the DDC112 to generate an offset current might
seem to also generate a lot of thermal noise. Surprisingly, in
this configuration the additional noise at the DDC112’s
output from the resistor is usually low, and in fact, decreases
as the values of the resistor increases. To understand why
this is, consider the two identical noise models of a resistor
shown in Figure 10. The one on the left is probably more
familiar and shows the resistor modeled as a voltage source
in series with a noiseless resistor. Considering only thermal
FIGURE 10. Equivalent Models for Thermal Noise of a
Resistor.
R
S
V
() = 4KTR
S
i
() =4R
R
FIGURE 9. Time Plot While Using the Evaluation Fixture as Configured in Figure 7 With No Input Signal.
(2)
v
S
H
d
2
0
2
= ∫
( )
( )
∞
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