LTC2471/LTC2473
APPLICATIONS INFORMATION
There is a limit to how large R • C should be for a given
ments, low impedance voltage source monitoring, and so
S
IN
application. Increasing R beyond a given point increases
on. The resultant INL vs V is shown in Figure 14. The
S
IN
the voltage drop across R due to the input current,
measurements of Figure 14 include a capacitor C
cor-
S
PAR
to the point that significant measurement errors exist.
Additionally, forsomeapplications, increasingtheR •C
respondingtoaminimumsizedlayoutpadandaminimum
width input trace of about 1 inch length.
S
IN
product too much may unacceptably attenuate the signal
at frequencies of interest.
Signal Bandwidth, Transition Noise and Noise
Equivalent Input Bandwidth
For most applications, it is desirable to implement C as
IN
2
TheLTC2471/LTC2473includeasinc typedigitalfilter.The
a high-quality 0.1μF ceramic capacitor and to set R ≤
S
first notch is located at 500Hz if the 250sps output rate is
selected and 2kHz if the 1ksps output rate is selected. The
calculated input signal attenuation vs. frequency over a
widefrequencyrangeisshowninFigure15.Thecalculated
input signal attenuation vs. frequency at low frequencies
is shown in Figure 16. The converter noise level is about
1k. This capacitor should be located as close as possible
+
–
to the actual IN , IN and IN package pins. Furthermore,
the area encompassed by this circuit path, as well as the
path length, should be minimized.
In the case of a 2-wire sensor that is not remotely
grounded, it is desirable to split R and place series
S
3μV
and can be modeled by a white noise source con-
RMS
resistors in the ADC input line as well as in the sensor
ground return line, which should be tied to the ADC GND
pin using a star connection topology.
nected at the input of a noise-free converter.
On a related note, the LTC2473 uses two separate A/D
converters to digitize the positive and negative inputs.
Figure 13 shows the measured LTC2473 INL vs Input
EachoftheseA/Dconvertershas3μV
transitionnoise.
RMS
Voltage as a function of R value with an input capacitor
S
If one of the input voltages is within this small transition
noise band, then the output will fluctuate one bit, regard-
less of the value of the other input voltage. If both of the
input voltages are within their transition noise bands, the
output can fluctuate 2 bits.
C = 0.1μF.
IN
Insomecases,R canbeincreasedabovetheseguidelines.
S
The input current is zero when the ADC is either in sleep
or I/O modes. Thus, if the time constant of the input RC
circuit τ = R • C , is of the same order of magnitude or
S
IN
Forasimplesystemnoiseanalysis,theV drivecircuitcan
IN
longer than the time periods between actual conversions,
then one can consider the input current to be reduced
correspondingly.
be modeled as a single-pole equivalent circuit character-
ized by a pole location f and a noise spectral density n .
i
i
If the converter has an unlimited bandwidth, or at least a
These considerations need to be balanced out by the input
bandwidth substantially larger than f , then the total noise
i
signal bandwidth. The 3dB bandwidth ≈ 1/(2πR C ).
contribution of the external drive circuit would be:
S IN
Finally, if the recommended choice for C is unacceptable
IN
Vn = ni π /2• fi
fortheuser’sspecificapplication,analternatestrategyisto
eliminateC andminimizeC andR .Inpracticalterms,
IN
PAR
S
Then, the total system noise level can be estimated as
2
thisconfigurationcorrespondstoalowimpedancesensor
directly connected to the ADC through minimum length
traces. Actual applications include current measurements
through low value sense resistors, temperature measure-
the square root of the sum of (V ) and the square of the
n
LTC2471/LTC2473 noise floor.
24713f
16