Texas Instruments | AN-233 The A/D Easily Allows Many Unusual Applications (Rev. B) | Application notes | Texas Instruments AN-233 The A/D Easily Allows Many Unusual Applications (Rev. B) Application notes

Texas Instruments AN-233 The A/D Easily Allows Many Unusual Applications (Rev. B) Application notes
Application Report
SNAA078B – September 1974 – Revised April 2013
AN-233 The A/D Easily Allows Many Unusual Applications
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ABSTRACT
Two design features of the Texas Instruments ADC0801 series of A/D converters provide for easy
solutions to many system design problems. The combination of differential analog voltage inputs and a
voltage reference input that can range from near zero to 5VDC are key to these application advantages.
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Contents
Accommodation of Arbitrary Analog Inputs ..............................................................................
Limits of VREF/2 Voltage Magnitude .......................................................................................
A 10-Bit Application .........................................................................................................
A Microprocessor Controlled Voltage Comparator .....................................................................
DACs Multiply and A/Ds Divide ...........................................................................................
Combine Analog Self-Test with Your Digital Routines .................................................................
Control Temperature Coefficients with Converters .....................................................................
Save an Op Amp ............................................................................................................
Digitizing a Current Flow ...................................................................................................
Conclusions ..................................................................................................................
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List of Figures
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Providing Arbitrary Zero and Span Accommodation ................................................................... 3
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Operating with a Ratiometric Transducer that Outputs 15% to 85% of VCC ......................................... 3
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Linearity Error for Reduced Analog Input Spans
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10-Bit A/D Using the 8-Bit ADC801
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Directly Encoding a Low Level Signal ...................................................................................
Digitizing a Current Flow ...................................................................................................
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SNAA078B – September 1974 – Revised April 2013
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1
Accommodation of Arbitrary Analog Inputs
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Accommodation of Arbitrary Analog Inputs
In many systems, the analog signal that has to be converted does not range clear to ground (0.00 VDC) nor
does it reach up to the full supply or reference voltage value. This presents two problems: 1) a “zerooffset” provision is needed—and this may be volts, instead of the few millivolts that are usually provided;
and 2) the “full scale” needs to be adjusted to accommodate this reduced span. (“Span” is the actual
range of the analog input signal, from VIN MIN to VIN MAX.) This is easily handled with the converter as shown
in Figure 1.
Note that when the input signal, VIN, equals VIN MIN the “differential input” to the A/D is zero volts and
therefore a digital output code of zero is obtained. When VINequals VIN MAX, the “differential input” to the
A/D is equal to the “span” (for reference applications convenience, there is an internal gain of two to the
voltage that is applied to pin 9, the VREF/2 input), therefore the A/D will provide a digital full scale. In this
way a wide range of analog input voltages can be easily accommodated.
An example of the usefulness of this feature is when operating with ratiometric transducers that do not
output the complete supply voltage range. Some, for example, may output 15% of the supply voltage for a
zero reading and 85% of the supply for a full scale reading. For this case, 15% of the supply should be
applied to the VIN(−) pin and the VREF/2 pin should be biased at one-half of the span, which is ½ (85%–15%)
or 35% of the supply. This properly shifts the zero and adjusts the full scale for this application. The VIN(−)
input can be provided by a resistive divider that is driven by the power supply voltage and the VREF/2 pin
should be driven by an op amp. This op amp can be a unity-gain voltage follower that also obtains an
input voltage from a resistive divider. These can be combined as shown in Figure 2.
This application can allow obtaining the resolution of a greater than 8-bit A/D. For example, 9-bit
performance with the 8-bit converter is possible if the span of the analog input voltage should only use
one-half of the available 0V to 5V span. This would be a span of approximately 2.5V that could start
anywhere over the range of 0V to 2.5VDC.
The RC network on the output of the op amp of Figure 2 is used to isolate the transient displacement
current demands of the VREF/2 input from the op amp.
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Limits of VREF/2 Voltage Magnitude
A question arises as to how small in value the span can be made. An ADC0801 part is shown in Figure 3
where the VREF/2 voltage is reduced in steps: from A), 2.5V (for a full scale reading of 5V); to B), 0.625V
(for a full scale reading of 1.25V—this corresponds to the resolution of a 10-bit converter over this
restricted range); to C), 0.15625V (for a full scale reading of 0.3125V—which corresponds to the
resolution of a 12-bit converter). Note that at 12 bits the linearity error has increased to ½ LSB.
For these reduced reference applications the offset voltage of the A/D has to be adjusted as the voltage
value of the LSB changes from 20 mV to 5 mV and finally to 1.25 mV as we go from A) to B) to C). This
offset adjustment is easily combined with the setting of the VIN MIN value at the VIN(−) pin.
Operation with reduced VREF/2 voltages increases the requirement for good initial tolerance of the
reference voltage (or requires an adjustment) and also the allowed changes in the VREF/2 voltage over
temperature are reduced.
An interesting application of this reduced reference feature is to directly digitize the forward voltage drop of
a silicon diode as a simple digital temperature sensor.
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AN-233 The A/D Easily Allows Many Unusual Applications
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Limits of VREF/2 Voltage Magnitude
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Figure 1. Providing Arbitrary Zero and Span Accommodation
Figure 2. Operating with a Ratiometric Transducer that Outputs 15% to 85% of VCC
Figure 3. Linearity Error for Reduced Analog Input Spans
SNAA078B – September 1974 – Revised April 2013
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3
A 10-Bit Application
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A 10-Bit Application
This analog flexibility can be used to increase the resolution of the 8-bit converter to 10 bits. The heart of
the idea is shown in Figure 4. The two extra bits are provided by the 2-bit external DAC (resistor string)
and the analog switch, SW1.
Note that the VREF/2 pin of the converter is supplied with ⅛ VREF so each of the four spans that are
encoded will be:
(1)
In actual implementation of this circuit, the switch would be replaced by an analog multiplexer (such as the
CD4066 quad bilateral switch) and a microprocessor would be programmed to do a binary search for the
two MS bits. These two bits plus the 8 LSBs provided by the A/D give the 10-bit data. For a particular
application, this basic idea can be simplified to a 1-bit ladder to cover a particular range of analog input
voltages with increased resolution. Further, there may exist a priori knowledge by the CPU that could
locate the analog signal to within the 1 or 2 MSBs without requiring a search algorithm.
Figure 4. 10-Bit A/D Using the 8-Bit ADC801
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AN-233 The A/D Easily Allows Many Unusual Applications
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A Microprocessor Controlled Voltage Comparator
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4
A Microprocessor Controlled Voltage Comparator
In applications where set points (or “pick points”) are set up by analog voltages, the A/D can be used as a
comparator to determine whether an analog input is greater than or less than a reference DC value. This
is accomplished by simply grounding the VREF/2 pin (to provide maximum resolution) and applying the
reference DC value to the VIN(−) input. Now with the analog signal applied to the VIN(+)input, an all zeros
code will be output for VIN(+) less than the reference voltage and an all ones code for VIN(+) greater than the
reference voltage. This reduces the computational loading of the CPU. Further, using analog switches, a
single A/D can encode some analog input channels in the “normal” way and can provide this comparator
operation, under microprocessor control, for other analog input channels.
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DACs Multiply and A/Ds Divide
Computation can be directly done with converter components to either increase the speed or reduce the
loading on a CPU. It is rather well known that DACs multiply—and for this reason many are actually called
“MDACs” to signify “multiplying DAC.” An analog product voltage is provided as an output signal from a
DAC for a hybrid pair of input signals—one is analog (the VREF input) and the other is digital.
The A/D provides a digital quotient output for two analog input signals. The numerator or the dividend is
the normal analog input voltage to the A/D and the denominator or the divisor is the VREF input voltage.
High speed computation can be provided external to the CPU by either or both of these converter
products. DACs are available that provide 4-quadrant multiplications (the MDACs and MICRO-DACs™),
but A/Ds are usually limited to only one quadrant.
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Combine Analog Self-Test with Your Digital Routines
A new innovation is the digital self-test and diagnostic routines that are being used in equipment. If an 8bit A/D converter and an analog multiplexer are added, these testing routines can then check all power
supply voltage levels and other set point values in the system. This is a major application area for the new
generation converter products.
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Control Temperature Coefficients with Converters
The performance of many systems can be improved if voltages within the system can be caused to
change properly with changes in ambient temperature. This can be accomplished by making use of low
cost 8-bit digital to analog converters (DACs) that are used to introduce a “dither” or small change about
the normal operating values of DC power supplies or other voltages within the system. Now, a single
measurement of the ambient temperature and one A/D converter with a MUX can be used by the
microprocessor to establish proper voltage values for a given ambient temperature. This approach easily
provides non-linear temperature compensation and generally reduces the cost and improves the
performance of the complete system.
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AN-233 The A/D Easily Allows Many Unusual Applications
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5
Save an Op Amp
8
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Save an Op Amp
In applications where an analog signal voltage that is to be converted may only range from, for example,
0VDC to 500 mVDC, an op amp with a closed-loop gain of 10 is required to allow making use of the full
dynamic range (0VDC to 5VDC) of the A/D converter. An alternative circuit approach is shown in Figure 5.
Here we, instead, attenuate the magnitude of the reference voltage by 10:1 and apply the 0 to 500 mV
signal directly to the A/D converter. The VIN(−) input is now used for a VOS adjust, and due to the “sampleddata” operation of the A/D there is essentially no VOS drift with temperature changes.
As shown in Figure 5, all zeros will be output by the A/D for an input voltage (at the VIN(+) input) of 0VDC
and all ones will be output by the A/D for a 500mVDC input signal. Operation of the A/D in this high
sensitivity mode can be useful in many low cost system applications.
Figure 5. Directly Encoding a Low Level Signal
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AN-233 The A/D Easily Allows Many Unusual Applications
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Digitizing a Current Flow
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Digitizing a Current Flow
In system applications there are many requirements to monitor the current drawn by a PC card or a high
current load device. This typically is done by sampling the load current flow with a small valued resistor.
Unfortunately, it is usually desired that this resistor be placed in series with the VCC line. The problem is to
remove the large common-mode DC voltage, amplify the differential signal, and then present the ground
referenced voltage to an A/D converter.
All of these functions can be handled by the A/D using the circuit shown in Figure 6. Here we are making
use of the differential input feature and the common-mode rejection of the A/D to directly encode the
voltage drop across the load current sampling resistor. An offset voltage adjustment is provided and the
VREF/2 voltage is reduced to 50 mV to accommodate the input voltage span of 100 mV. If desired, a
multiplexer can be used to allow switching the VIN(−) input among many loads.
Figure 6. Digitizing a Current Flow
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Conclusions
At first glance it may appear that the A/D converters were mainly designed for an easy digital interface to
the microprocessor. This is true, but the analog interface has also been given attention in the design and a
very useful converter product has resulted from this combination of features.
SNAA078B – September 1974 – Revised April 2013
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AN-233 The A/D Easily Allows Many Unusual Applications
Copyright © 1974–2013, Texas Instruments Incorporated
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