Questions about optimizing Sensor outputs to dsp/microcontroller A/D inputs

[steve]

Ok, when hooking up a sensor output to a/d input we want to minimize
the ratio of RMS noise to the LSB value. I would think we want a
sensor with a high voltage output and a A/D with a high voltage input
so that the LSB is well above the noise floor. Stand alone a/d's
usually have nice high input ranges (e.g., Maxim 1271 with a +/- 10
Volt input), but many modern DSP and microcontroller A/D inputs are
usually +5 volts and the really modern and attractive ones are in the
+2.5 Volts range (Analog device ADUC7X series, Cypress PSoC ), which
initially look unattractive as far as the A/D is concerned.

However, when looking at the Maxim datasheet, although its take +/- 10
Volt input the internal reference is only 2.5 Volts, which is then
scaled up interally to 4.096V. Secondly, many sensors with high
voltage outputs simply have an internal final stage amp that scales
the output voltage. So my questions are

1) Are those 2.5 Volt A/D's really at a disadvantage compared to the
wide range stand alone Maxim type A/D's since they really work off 2.5
volt references? I think the only purpose of the wide range inputs is
to eliminate the need to externally rescale large inputs, not to
decrease the RMS noise to LSB ratio. (which is what I thought at first
glance)
2) When Maxim scales up the 2.5 volts reference to 4.096 volts, what
the purpose of that? I would think that the amp would amplify the
noise too (unless its a differential amp). Or maybe they just want 1
count = 1 mV?
3) Sensors with 5V outputs typically are just scaled up to 5 Volts
internally, and, with external parts, can be rescaled to 2.5 volts. If
I do that am I hurting the noise/lsb ratio? I would think that if they
are scaling up the output they are also scaling up the noise too
(again unless they are using differential amps).

If there is a application note somewhere about this let me know
please!
thanks
steve

 

[Ian Stirling]

Ok, when hooking up a sensor output to a/d input we want to minimize
the ratio of RMS noise to the LSB value. I would think we want a
sensor with a high voltage output and a A/D with a high voltage input
so that the LSB is well above the noise floor. Stand alone a/d's
usually have nice high input ranges (e.g., Maxim 1271 with a +/- 10
Volt input), but many modern DSP and microcontroller A/D inputs are

1) Are those 2.5 Volt A/D's really at a disadvantage compared to the
wide range stand alone Maxim type A/D's since they really work off 2.5
volt references? I think the only purpose of the wide range inputs is
to eliminate the need to externally rescale large inputs, not to
decrease the RMS noise to LSB ratio. (which is what I thought at first
glance)

http://www.maxim-ic.com/

Look at number of bits vs supply voltage.

 

[Randy Yates]

Hi Steve,

I'm not an expert, but you usually want to scale your analog input so
that its full-scale value matches the full-scale value of the
A/D. That way you use the full dynamic range of the converter.

Also, note that gain scaling at this stage (pretty much near the end
of the analog processing chain) usually doesn't affect the analog SNR
too much. (Rather, it is the early gain stages that degrade SNR, or
noise figure, the most.) This is discussed, e.g., at

http://www.rfcafe.com/references/electrical/noise_figure.htm

Finally, note that if the analog SNR is worse than the digital
SNR (i.e., about 6N dB, where N is the number of bits in the A/D),
then that's OK - the lower bits will just be toggling with the
noise. I.e., you STILL want to match the full-scale analog to the
full-scale A/D input level, otherwise you'd be degrading the SNR
needlessly.

Hopefully there was something here you didn't already know.

--Randy

Ok, when hooking up a sensor output to a/d input we want to minimize
the ratio of RMS noise to the LSB value. I would think we want a
sensor with a high voltage output and a A/D with a high voltage input
so that the LSB is well above the noise floor. Stand alone a/d's
usually have nice high input ranges (e.g., Maxim 1271 with a +/- 10
Volt input), but many modern DSP and microcontroller A/D inputs are
usually +5 volts and the really modern and attractive ones are in the
+2.5 Volts range (Analog device ADUC7X series, Cypress PSoC ), which
initially look unattractive as far as the A/D is concerned.

However, when looking at the Maxim datasheet, although its take +/- 10
Volt input the internal reference is only 2.5 Volts, which is then
scaled up interally to 4.096V. Secondly, many sensors with high
voltage outputs simply have an internal final stage amp that scales
the output voltage. So my questions are

1) Are those 2.5 Volt A/D's really at a disadvantage compared to the
wide range stand alone Maxim type A/D's since they really work off 2.5
volt references? I think the only purpose of the wide range inputs is
to eliminate the need to externally rescale large inputs, not to
decrease the RMS noise to LSB ratio. (which is what I thought at first
glance)
2) When Maxim scales up the 2.5 volts reference to 4.096 volts, what
the purpose of that? I would think that the amp would amplify the
noise too (unless its a differential amp). Or maybe they just want 1
count = 1 mV?
3) Sensors with 5V outputs typically are just scaled up to 5 Volts
internally, and, with external parts, can be rescaled to 2.5 volts. If
I do that am I hurting the noise/lsb ratio? I would think that if they
are scaling up the output they are also scaling up the noise too
(again unless they are using differential amps).

If there is a application note somewhere about this let me know
please!
thanks
steve

--
% Randy Yates % "I met someone who looks alot like you,
%% Fuquay-Varina, NC % she does the things you do,
%%% 919-577-9882 % but she is an IBM."
%%%% <[email protected]> % 'Yours Truly, 2095', *Time*, ELO
http://home.earthlink.net/~yatescr

 

[Ban]

Ok, when hooking up a sensor output to a/d input we want to minimize
the ratio of RMS noise to the LSB value. I would think we want a
sensor with a high voltage output and a A/D with a high voltage input
so that the LSB is well above the noise floor. Stand alone a/d's
usually have nice high input ranges (e.g., Maxim 1271 with a +/- 10
Volt input), but many modern DSP and microcontroller A/D inputs are
usually +5 volts and the really modern and attractive ones are in the
+2.5 Volts range (Analog device ADUC7X series, Cypress PSoC ), which
initially look unattractive as far as the A/D is concerned.

However, when looking at the Maxim datasheet, although its take +/- 10
Volt input the internal reference is only 2.5 Volts, which is then
scaled up interally to 4.096V. Secondly, many sensors with high
voltage outputs simply have an internal final stage amp that scales
the output voltage. So my questions are

1) Are those 2.5 Volt A/D's really at a disadvantage compared to the
wide range stand alone Maxim type A/D's since they really work off 2.5
volt references? I think the only purpose of the wide range inputs is
to eliminate the need to externally rescale large inputs, not to
decrease the RMS noise to LSB ratio. (which is what I thought at first
glance)

Somehow your opinion seems possible, but it is important to consider the
amount of each individual component in your accuracy Start with the A/D
itself. It has a specified accuracy included the effect of the reference
voltage scaler and other factors (temp. range) which is expressed in for
example +/-1LSB, which in your case would be 2mV.
Now also your sensor has a certain noise level which is amplified by the
signal conditioning. Lets say the voltage noise be 500nV/Hz^-2 at the
0...4.096V output. With a bandwidth of 10kHz this would be 50uVrms, the
peaks might be 6 times higher, which is still only 0.3 LSB . So up- and
downscaling is not of much impact because the converter and sensor noise is
dominant.

2) When Maxim scales up the 2.5 volts reference to 4.096 volts, what
the purpose of that? I would think that the amp would amplify the
noise too (unless its a differential amp). Or maybe they just want 1
count = 1 mV?
3) Sensors with 5V outputs typically are just scaled up to 5 Volts
internally, and, with external parts, can be rescaled to 2.5 volts. If
I do that am I hurting the noise/lsb ratio? I would think that if they
are scaling up the output they are also scaling up the noise too
(again unless they are using differential amps).

If there is a application note somewhere about this let me know
please!
thanks
steve

If you do not use standard exchangable sensors with +/-5 or 10V range, you
can better get an unconditioned sensor and build a dedicated amp + filter to
drive the 0...+2.5V analog input. This can be done from a 3.3V supply which
has become standard by now. You can use modern low voltage CMOS parts, needs
some input protection as shown in the data-sheet.

 

[Bob]

Ok, when hooking up a sensor output to a/d input we want to minimize
the ratio of RMS noise to the LSB value. I would think we want a
sensor with a high voltage output and a A/D with a high voltage input
so that the LSB is well above the noise floor. Stand alone a/d's
usually have nice high input ranges (e.g., Maxim 1271 with a +/- 10
Volt input), but many modern DSP and microcontroller A/D inputs are
usually +5 volts and the really modern and attractive ones are in the
+2.5 Volts range (Analog device ADUC7X series, Cypress PSoC ), which
initially look unattractive as far as the A/D is concerned.

However, when looking at the Maxim datasheet, although its take +/- 10
Volt input the internal reference is only 2.5 Volts, which is then
scaled up interally to 4.096V. Secondly, many sensors with high
voltage outputs simply have an internal final stage amp that scales
the output voltage. So my questions are

1) Are those 2.5 Volt A/D's really at a disadvantage compared to the
wide range stand alone Maxim type A/D's since they really work off 2.5
volt references? I think the only purpose of the wide range inputs is
to eliminate the need to externally rescale large inputs, not to
decrease the RMS noise to LSB ratio. (which is what I thought at first
glance)

The wide range parts (+- 10v) trend to be the older parts. Newer parts tend
to target the lower voltages used in battery powered or high speed circuts.
Noise-to-LSB ratio is not a particularly usefull way of looking at these
things. See below.

2) When Maxim scales up the 2.5 volts reference to 4.096 volts, what
the purpose of that? I would think that the amp would amplify the
noise too (unless its a differential amp). Or maybe they just want 1
count = 1 mV?

Don't worry about what they do with the reference. Worry about your error
budget.

3) Sensors with 5V outputs typically are just scaled up to 5 Volts
internally, and, with external parts, can be rescaled to 2.5 volts. If
I do that am I hurting the noise/lsb ratio? I would think that if they
are scaling up the output they are also scaling up the noise too
(again unless they are using differential amps).

Your system design should start with a specification of the overall
performance. From there you can begin chosing parts and calculating your
error budget. Broadband noise is only one part of that budget. Sample rates,
gain and offset drift, and linearity are also important (or not, depending
on wht you are trying to do). Sometimes noise > 1LSB is a good thing (google
"dither"). Don't forget to analyze any averaging or other digital filtering
effects on your signal.

If there is a application note somewhere about this let me know
please!
thanks
steve

There are lots of application notes. The key word here is "application". All
of this issues depend on what the *application* is. So, what's your
application?

Bob

 

[steve]

The wide range parts (+- 10v) trend to be the older parts. Newer parts tend
to target the lower voltages used in battery powered or high speed circuts.
Noise-to-LSB ratio is not a particularly usefull way of looking at these
things. See below.

Don't worry about what they do with the reference. Worry about your error
budget.


Your system design should start with a specification of the overall
performance. From there you can begin chosing parts and calculating your
error budget. Broadband noise is only one part of that budget. Sample rates,
gain and offset drift, and linearity are also important (or not, depending
on wht you are trying to do). Sometimes noise > 1LSB is a good thing (google
"dither"). Don't forget to analyze any averaging or other digital filtering
effects on your signal.


There are lots of application notes. The key word here is "application". All
of this issues depend on what the *application* is. So, what's your
application?

Bob

The question was application independent, namely, how to optimize the
sensor to a/d interface for best noise performance.

I "worry" about the internal workings of a chip because it gives you
insight concerning how the worse case specs were derived, and more
importantly, how to exploit them to create a product that is lower in
cost and higher in performance then the competition.

Yes I am sure if I blindly pick parts base solely on the req spec and
then verify the overall design via a worse case tolerance analysis I
will develop a reliable compliant product, but it may a product that
is too costly, too heavy, too power hungry or too impractical that no
one would buy it. For instance, the dither technique you mentioned
(oversample/average) when implemented may well satisfy an overall
performance requirement, but if I have to upgrade to 64x faster A/D
and increase the DSP clock by 4x to implement it, I would consider
that a poor design if I could of just fixed the cause of the problem
in the first place (i.e., A/D to sensor mismatch).

steve

 

[Paul Keinanen]

Ok, when hooking up a sensor output to a/d input we want to minimize
the ratio of RMS noise to the LSB value. I would think we want a
sensor with a high voltage output and a A/D with a high voltage input
so that the LSB is well above the noise floor. Stand alone a/d's
usually have nice high input ranges (e.g., Maxim 1271 with a +/- 10
Volt input), but many modern DSP and microcontroller A/D inputs are
usually +5 volts and the really modern and attractive ones are in the
+2.5 Volts range (Analog device ADUC7X series, Cypress PSoC ), which
initially look unattractive as far as the A/D is concerned.

What is the output impedance of the sensor ? If both the output
voltage and impedance is high, then the scaling would have to be done
with a voltage divider with high resistors. In extreme cases the
voltage noise from the voltage divider can be significant.

Also look at the input structure of the intended ADC. If it is some
kind of switched capacitance type, the input impedance is not
constant, but varies during each clock cycle, thus a high resistance
voltage divider can be a problem.

What is the needed signal bandwidth. With a high resistance voltage
divider, the stray capacitances from ADC input to ground will
attenuate high frequencies. In this case a capacitor is needed across
the upper resistor or even a proper capacitive voltage divider
parallel with the resistors (as in oscilloscope probes) to flat out
frequency response. However, a capacitive voltage divider will also
load the sensor at higher frequencies, so the sensor behaviour should
be checked in these cases also.

Of course, these are not very real issues, if the sensor output
impedance is low.

What is the distance between the sensor and ADC ? Is the distance
long or there is a risk of interference connected to the lines or
expect ground potential differences ? Assuming the sensor output
impedance is low and output voltage swing is large, placing the
voltage divider close to the ADC would be a good thing (compared to
using a sensor with a small voltage swing), since the interfering
voltage would also be attenuated. Select the voltage divider chain
total resistance to be the lowest value supported by the sensor
output.

Paul

 

[Bernhard Holzmayer]

I assume the resistor your
talking about is for current limiting to protect the A/D.

No. If it's a proper design and sensor connection is persistent,
there's no need to protect the A/D. If there's a socket/connector
pair on a removable cable, this might be an aspect.

This resistor is only something that I've become used to and just
for convenience:
I never directly connect two parts of a design unless unavoidable.

If there's a resistor, this resistor can easily be removed, which
results in two isolated, testable parts of my circuit.
If there wouldn't be a resistor, isolating both parts might be
difficult without damaging the board.

If I have the choice, I insert a 0805 SMD resistor with 0 Ohms for
this purpose. This one can easily be removed with the soldering
iron, and later on, even a spot of solder would repair it.

Bernhard

 

[steve]

If there's a resistor, this resistor can easily be removed, which
results in two isolated, testable parts of my circuit.
If there wouldn't be a resistor, isolating both parts might be
difficult without damaging the board.

If I have the choice, I insert a 0805 SMD resistor with 0 Ohms for
this purpose. This one can easily be removed with the soldering
iron, and later on, even a spot of solder would repair it.

Bernhard

Ok, I understand, yes that's a great idea for debug.
steve