20 Lesson Op-Amps #5 (Section 4-5) (CLO 4-3) This lesson is dedicated to Op-Amp application, in particular, D/A and Comparator circuits. The next two lessons are reserved for Instrumentation applications. Digital-to-Analog Converters (DACs or D/As) We will look at two different types of D/A circuits the weighted summer and the R-2R ladder circuit. The first is very easy for students to grasp since it is simply an inverting summer. The figure below is for a 4-bit DAC, but generally, DACs are eight or more bits in length. RF b b b VREF b1 + 2 + 3 + 4 VREF The output is given by vO = R 2 4 8 v1 R Where each bit drives an electronic switch that switches b1 between ground (0V) and VREF, whatever the designer v2 2R RF wants it to be. The simplicity of this design is offset by the accuracy that each resistor must have to make all the b2 vO bits weighing proportional. In general, this is not the v3 4R way DACs are designed.
b3
Consider instead the R-2R realization shown in the following circuit. Here the importance is that all resistors b4 be the same R or 2R. An easy task especially if laid-out on a PC board or thick- or thin-film construction. It is worthwhile to let the students derive the output for a simple 4-bit DAC and they can then readily expand it to an n-bit device. The analysis is best done by finding the Thvenin equivalent circuit using superposition. A RF vO Using the look-back method we can R R R readily see that RT = R. To find vT we 2R 2R 2R 2R 2R start by setting all of the sources v4 v3 v2 v1 except v1, the MSB (most significant bit) to zero. The resistors to the left LSB MSB (a) collapse thusly, the furthermost 2R is in parallel with the 2R that was in RT =R A RF vO series with v yielding simply R. This 4 equivalent R then is in series with the fist R between the v4 and v3 branch. vT This then produces 2R. This new 2R equivalent resistor is in parallel with (b) the 2R that is in series with the v3shorted source yielding R. Following this process to node A produces a single 2R connected between node A and ground. Then using a voltage divider, the contribution of the MSB is v1/2 at node A. Have the students RF v v v v1 + 2 + 3 + 4 . The do the remaining three sources. The result then is vO = 2R 2 4 8 realization in the text allows one to set the input voltage VREF to regardless of the values of the four bits.
v4 8R
Comparators Comparators are not linear devices, so one might argue that they do not belong in this lecture. We usually involve Op-Amps in their linear mode between VCC. However, comparators purposely let the output to saturate the Op-Amp resulting in just two output values, namely VCC. The comparator shown below is simply an Op-Amp without feedback. Hence, even the smallest input will drive the Op-Amp towards saturation since the open-loop gain is generally very large, >105. If we input the vP vO signal shown, the output will go positive to +VCC whenever the vN signal is positive and towards VCC whenever the signal goes vS negative. The output is also shown in the figure. This configuration is called a zero-crossing detector because the OpAmps output switches whenever the signal crosses zero volts. Ask the students how the output would change if the signal went to the terminal and the + terminal was grounded. Likewise, one can add a DC source VDC between vN and 5 ground. This would in effect cause the output to switch between +VCC and VCC whenever the signal exceeded 0 t VDC. Another useful application is to compare two signals (a) one connected to vN and the other to vP and to switch -5 whenever one signal exceeds the other. Such an V cc application could be used to monitor the temperature rise in an engine versus an ideal maximum temperature t 0 rise and to issue a warning whenever the actual signal (b) exceeds the ideal maximum rise. -Vcc The last application would be as a flash Analog-to-Digital Converter (A/D or ADC). The figure below shows how a 3-bit device would be connected. The voltage divider sets the switching values for the input. Whenever the input + 8V exceeds 1 V vO1 goes high. When it exceeds 3 V both vO1 and vO2 go high and when the input exceeds 5 V all three 3R outputs are high. You might compare this design with the vO3 one in the text. Since the output occurs instantaneously 2R vN3 with the input changing, it is called a Flash converter. v O2 These are the fastest of all A/D converters. However, most A/Ds today are not Flash converters but rather sample-hold 2R vN2 devices. We will talk about these after we have studied vO1 capacitors and RC step response.
vS
R
vN1 VOH = 5 V VOL = 0 V