"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10
"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10
"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10
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12-4<br />
(A)<br />
4 V<br />
3 V ADC<br />
0 V<br />
Sensor<br />
Output Span<br />
1 V<br />
ADC<br />
Input<br />
Span<br />
2 V<br />
0 V<br />
Sensor<br />
Output Span<br />
(B)<br />
4 V<br />
0 V<br />
ADC<br />
Input<br />
Span<br />
Figure 12–3. Example of Spans That Require Correction<br />
2 V<br />
0 V<br />
Sensor<br />
Output Span<br />
(C)<br />
4 V<br />
1 V<br />
Input<br />
Span<br />
<strong>The</strong> output span of <strong>the</strong> transducer must be matched to <strong>the</strong> <strong>in</strong>put span of <strong>the</strong> ADC to<br />
achieve optimum performance. When <strong>the</strong> spans are mismatched ei<strong>the</strong>r <strong>the</strong> transducer<br />
output voltage does not fit <strong>in</strong>to <strong>the</strong> ADC <strong>in</strong>put span thus los<strong>in</strong>g sensor data, or <strong>the</strong> transducer<br />
output voltage does not fill <strong>the</strong> ADC <strong>in</strong>put span thus los<strong>in</strong>g ADC accuracy. <strong>The</strong> latter<br />
situation requires an <strong>in</strong>crease <strong>in</strong> ADC dynamic range (<strong>in</strong>creased cost) because a higher<br />
bit converter must be used to achieve <strong>the</strong> same resolution. <strong>The</strong> best analog circuit available<br />
for match<strong>in</strong>g <strong>the</strong> spans is <strong>the</strong> op amp because it level shifts and amplifies <strong>the</strong> <strong>in</strong>put<br />
voltage to make <strong>the</strong> spans equal. <strong>The</strong> op amp is so versatile that it shifts <strong>the</strong> signal’s dc<br />
level and amplifies <strong>the</strong> <strong>in</strong>put signal simultaneously.<br />
A similar but different problem exists <strong>in</strong> <strong>the</strong> digital to analog converter (DAC) to actuator<br />
<strong>in</strong>terface. <strong>The</strong> DAC output voltage or current span must match <strong>the</strong> actuator <strong>in</strong>put voltage<br />
span to achieve maximum performance. <strong>The</strong> procedure for match<strong>in</strong>g <strong>the</strong> DAC output<br />
span to <strong>the</strong> actuator <strong>in</strong>put span can be quite different from <strong>the</strong> procedure for match<strong>in</strong>g <strong>the</strong><br />
transducer output span to <strong>the</strong> ADC <strong>in</strong>put span. Transducer outputs are usually low-level<br />
signals, thus care must be taken to preserve <strong>the</strong>ir signal to noise ratio. Actuator <strong>in</strong>put signals<br />
may require significant power, thus robust op amps are required to drive some actuators.<br />
<strong>The</strong> system specifications eventually determ<strong>in</strong>e <strong>the</strong> transducer, ADC, and analog circuit<br />
specifications. System specifications are seen as absolute specifications; <strong>the</strong>y must be<br />
met for <strong>the</strong> design to function <strong>in</strong> a satisfactory manner. Component specifications are divided<br />
<strong>in</strong>to several categories; absolute maximum rat<strong>in</strong>gs (AMR), guaranteed m<strong>in</strong>imum/<br />
maximum specifications (VMAX or VMIN), typical specifications (V), and guaranteed but<br />
not tested specifications (GNT).<br />
If any of <strong>the</strong> device parameters are taken beyond <strong>the</strong> AMR, <strong>the</strong> device can be destroyed<br />
(expect destruction). <strong>The</strong> manufacturer guard bands <strong>the</strong> AMR to guarantee safety and<br />
quality, and you should guard band <strong>the</strong> AMRs, too. Typical specifications are <strong>the</strong> most<br />
appeal<strong>in</strong>g, but throw <strong>the</strong>m out because <strong>the</strong>y are mean<strong>in</strong>gless <strong>in</strong> most cases. In <strong>the</strong> vast<br />
majority of cases <strong>the</strong> typical specifications are not related to mean<strong>in</strong>gful data; ra<strong>the</strong>r, <strong>the</strong>y<br />
are market<strong>in</strong>g dreams. Never design with typical specifications unless you are <strong>in</strong> <strong>the</strong> habit<br />
of design<strong>in</strong>g with mean<strong>in</strong>gless data or have a good reason for believ<strong>in</strong>g that typical is<br />
close to reality. A violation of this rule is a specification like output voltage sw<strong>in</strong>g that de-