AN018: Do's and Don'ts of Applying A/D Converters

Do’s and Don’ts of Applying A/D Converters
Application Note
AN018
Introduction
in the resistance of the common lead will generate a DC
offset voltage. Even the auto-zero circuit of an integrating
A/D converter cannot remove this error. But in addition, this
current will have several varying components. The clock
oscillator, and the various digital circuits driven from it, will
show supply current variations at the clock frequency, and
usually at submultiples also. For a successive approximation
converter, these will cause an additional effective offset. For
an integrating converter, at least the high-frequency
components should average out. In some converters, the
analog supply currents will also vary with the clock (or a submultiple) frequency. If the display is multiplexed, that current
will vary with the multiplex frequency, usually some fraction
of the clock frequency. For an integrating converter, both
digital and analog section currents will change as the
converter goes from one phase of conversion to another.
(Currents of this type injected into an auto-zero loop are
particularly obstinate.) Another serious source of variation is
the change in digital and display section currents with the
result value. This frequently shows up as an oscillating
result, and/or missing results; one value being displayed
displaces the effective input to a new value, which is
converted and displayed, leading to a different displacement,
a new value and so on. This sequence usually closes after
two or three values, which are displayed in sequence.
In many applications, the limitation in the performance of a
system lies in how the individual components are used. The
Analog-to-Digital Converter (A/D) can also be considered as
a component and, therefore, proper design procedures are
necessary to obtain the optimum accuracy. Intersil A/D
converters are inherently extremely accurate devices. To
obtain the optimum performance from them, care should be
taken in the hookup and choice of external components
used. Test equipment used in system evaluation should be
substantially more accurate and stable than the system
needs to be. The following sections illustrate do’s and don’ts
to obtain the best results from any system.
Don’t Introduce Ground Loop Errors
Plan your grounding carefully. Probably the most common
source of error in any Analog-Digital system is improper
grounding. Let’s look at Figure 1. All the grounds are tied
together, so everything should be OK, right? Wrong! Almost
everything is wrong with this connection.
The power supply currents for the analog and digital
sections, together with the output or display currents, all flow
through a lead common to the input. Let us analyze some of
the errors we have introduced. The average currents flowing
OUTPUT,
DISPLAY,
ETC.
I/P HI
ANALOG
SECTION
DIGITAL
SECTION
DIG
GND
AN
GND
AN
PS
DIG
PS
CLOCK
OSC
IDIG
IAN
I/P LO
IDISPLAY
ANALOG
POWER
SUPPLY
DIGITAL
POWER
SUPPLY
FIGURE 1. DON’T HOOK IT UP LIKE THIS!
1
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Application Note 018
of the converter. And coupling from any digital line into a
high impedance input line can lead to errors in any system.
A more subtle source of errors in this circuit comes from the
clock oscillator frequency. For an integrating converter,
variations in clock frequency during a single conversion
cycle due to varying digital supply voltage or supply currents,
or ground loops to a timing capacitor, will lead to incorrect
results.
Do Use Adequate Quality Components
For successive approximation converters, the resistors used
must have excellent time and temperature stability to
maintain accuracy. Any adjustment potentiometers, etc.,
must be of compatible quality (note that in some trimpots,
the slider position moves with temperature!).
Figure 2 shows a much better arrangement. The digital and
analog grounds are connected by a line carrying only the
interface currents between sections, and the input section is
also tied back by a low current line. The display current loop
will not affect the analog section and the clock section is
isolated by a decoupling capacitor. Note that external
reference return currents and any other analog system
currents must also be returned carefully to analog ground.
For dual slope converters, the component selection is less
critical. Long term drifts in the integrating resistor and the
capacitors are not important. However, any resistive divider
used on the reference, especially if it is adjustable, must be
of sufficient stability not to degrade system accuracy.
Dielectric absorption in the integrating capacitor is important
and the integrating resistor must have a negligible voltage
coefficient to ensure linearity [1-7]. Noisy components will
lead to noisy performance, whether in the integrator,
autozero or clock circuits.
Don’t Couple Digital Signals Into
Analog Lines
Although Intersil’s A/D converter circuits have been designed
to minimize the internal coupling of digital signals into analog
lines, the external capacitive coupling is controlled by the
user. For best results, it is advisable to keep analog and
digital sections separated on PC boards. A few examples of
the results of capacitive coupling follow.
Do Use a Good Reference
Good references are like good wines; nobody is quite sure
how to make them but generally the older the technology
used, the better the result, and the proof lies in the tasting
(or testing). Thus, it is hard to beat the old temperature
compensated zener with the current flow adjusted to the
optimum for each diode. If you aren’t into Zinfandel Superior
Premier Cru (1972), the Intersil ICL8052 has a fairly good
reference built in. In either case, the division down to the
desired reference voltage requires care also (see above).
And it is a fundamental fact that no converter can be better
than its reference voltage.
On dual slope converters, the “busy” line swings from one
state to the other at the end and beginning of the autozero
cycle. Capacitive coupling from this line to the autozero or
integrating capacitors will induce an effective input offset
voltage. A similar effect occurs with the “Measure/Zero” line
on charge-balancing converters and for a successive
approximation converter with coupling between “end of
conversion” and a sample-and-hold capacitor. For a
multiplexed display device, coupling between the multiplexed
or “digit” lines and these capacitors can lead to nonlinearity
OUTPUT,
DISPLAY,
ETC.
I/P HI
ANALOG
SECTION
AN
PS
I/P LO
AN
GND
ANALOG
POWER
SUPPLY
DIGITAL
SECTION
DIG
GND
DIG
PS
CLOCK
OSC
DIGITAL
POWER
SUPPLY
FIGURE 2. DO HOOK IT UP LIKE THIS!
2
Application Note 018
Do Watch Out for Thermal Effects
All integrated circuits have thermal time constants of a few
milliseconds to dissipation changes in the die. These can
cause changes in such parameters as offset voltages and
VBE matching. For example, the power dissipation in an
8018 quad current switch depends on the digital value.
Although the die is carefully designed to minimize the effects
of this, the resultant temperature changes will affect the
matching between current switch values to a small degree.
Inappropriate choice of supply voltages and current levels
can enhance these differences, leading to errors. Similarly,
the power dissipated in a dual-slope converter circuit
depends on the comparator polarity and hence varies during
the conversion cycle. Offset voltage variations due to this
cannot be autozeroed out, and so can lead to errors. Again a
poor choice of comparator loading or swing will enhance this
(normally) minor effect. The power dissipation in an output
display could be coupled into the sensitive analog sections
of a converter, leading to similar problems. And thermal
gradients between IC packages and PC boards can lead to
thermoelectric voltage errors in very sensitive systems.
Do Use the Maximum Input Scale
To minimize all other sources of error, it is advisable to use
the highest possible full scale input voltage. This is
particularly important with successive approximation
converters, where offset voltage errors can quickly get above
1 LSB, but even for integrating-type converters, noise and
the various other errors discussed above, will increase in
importance for lower-than-maximum full scale ranges.
Preconverter gain is usually preferable for small original
signals. All Intersil’s integrating converters have a digital
output line that can be used to extend autozero to
preconditioning circuits (being careful not to couple the
digital signal into the analog system, of course).
Do Check These Areas
Tie digital inputs down (or up) if you are not using them. This
will prevent stray input spikes from affecting operation.
Bypass all supplies with a large and a small capacitor close
to the package. Limit input currents into any IC pin to values
within the maximum rating of the device (or a few mA if not
specified) to avoid damaging the device. Ensure that power
supplies do not reverse polarity or spike to high values when
turned on or off. Remember that many digital gates take
higher-than-normal supply currents for inputs between
defined logic levels. Remember also that gates can look like
amplifiers under these circumstances. An example is shown
in Figure 3, where stray and internal input-to-output
capacitance is multiplied by the gain of the gate just at the
threshold causing a large effective load capacitance on the
ICL8052 comparator. A non-inverting gate here could lead to
oscillations.
3
COMPARATOR
ICL8052
FIGURE 3.
External Adjustment Procedure
Most of the A/D converters now offered by Intersil do not
require an offset adjustment. They have internal autozero
circuits which typically give less than 10µV offset. Therefore,
the only optional adjustment required to obtain optimum
accuracy in a given application is the full scale or gain
reading.
With the A/D converter in a continuous mode of conversion,
the following procedure is recommended: The full scale
adjustment is made by setting the input voltage to precisely
1/ LSB less than full scale or 1/ LSB down from nominal
2
2
full scale. (Note that the nominal full scale is actually never
reached but is always one LSB short.) Adjust the full scale
control until the converter output just barely switches from
full output to one count less than full output.
References
For Intersil documents available on the internet, see web site
http://www.intersil.com/
Intersil AnswerFAX (321) 724-7800.
[1] AN016 Application Note, Intersil Corporation, “Selecting
A/D Converters”, AnswerFAX Doc. No. 9016.
[2] AN017 Application Note, Intersil Corporation, “The
Integrating A/D Converter”, AnswerFAX Doc. No. 9017.
[3] AN023 Application Note, Intersil Corporation, “Low Cost
Digital Panel Meter Designs and Complete Instructions
for LCD and LED Kits”, AnswerFAX Doc. No. 9023.
[4] AN028 Application Note, Intersil Corporation, “Building
On Autoranging DMM with the ICL7103A/8052A A/D
Converter Pair”, AnswerFAX Doc. No. 9028.
[5] AN030 Application Note, Intersil Corporation, “ICL7104,
A Binary Output A/D Converter for µProcessors”,
AnswerFAX Doc. No. 9030.
[6] AN047 Application Note, Intersil Corporation, “Games
People Play with Intersil’s A/D Converters”, AnswerFAX
Doc. No. 9047.
[7] AN052 Application Note, Intersil Corporation, “Tips for
Using Single Chip 3-1/2 Digit A/D Converters”,
AnswerFAX Doc. No. 9052.
Application Note 018
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