Differences

This shows you the differences between two versions of the page.

--- university:courses:electronics:electronics-lab-nr [11 Feb 2013 15:48] – created Doug Mercer
+++ university:courses:electronics:electronics-lab-nr [27 Mar 2017 16:47] – [Materials:] Doug Mercer
@@ Line 1: / Line 1: @@
-====== Activity: Generating a Negative Voltage Reference Without Precision Resistors ======
+======Activity: Generating a Negative Voltage Reference======
-===== Objective: =====
+=====Objective:=====
-The objective of this activity is to investigate ways to create negative reference voltages using positive voltage references or regulators without needing to rely on precision matched resistors for accuracy.
+The objective of this lab activity is to investigate ways to produce negative reference voltages. Positive voltage references or regulator configurations are more commonly available. Conventional methods of generating a negative reference voltage from a positive voltage involve inverting op-amp stages which tend to rely on precision matched resistors for accuracy.
-===== Background: =====
+=====Background:=====
-The obvious method for generating a negative reference voltage from a positive reference voltage is to simply use an inverting op amp as shown in figure 1(a). This approach requires two precision matched resistors, R<sub>1</sub> and R<sub>2</sub>. Errors in the matching, in addition to any offset voltage in the op amp, will produce errors at the negative output -V<sub>REF</sub>. The circuit described in this activity, shown in figure 1(b), is used to generate a negative reference voltage without the use of precision matched resistors, thereby providing higher accuracy with fewer components.
+In figure 1(a) the simple zener diode circuit, consisting of R<sub>Z</sub> and D<sub>Z</sub> from the zener diode regulator lab activity[1], is used to produce a  positive reference voltage, +V<sub>REF</sub>. In a positive voltage reference a non-inverting op-amp buffer is often included to scale the output voltage and supply any current needed in the load. The obvious method for generating a negative reference voltage is to instead use an inverting op-amp stage as shown in the figure. This approach requires two precision matched resistors, R<sub>1</sub> and R<sub>2</sub>. Errors in the matching, in addition to any offset voltage in the op-amp, will produce errors at the negative output -V<sub>REF</sub>. However, one potential side benefit of this inverting amplifier configuration is that -V<sub>REF</sub> need not have the same absolute value as +V<sub>REF</sub>. The negative reference voltage can be scaled up or down by altering the ratio of R<sub>1</sub> and R<sub>2</sub>.
+The alternate configuration we will be investigating in this lab activity is shown in figure 1(b). It generates a negative reference voltage without the dependence on ratio matched resistors, potentially providing higher accuracy with fewer components.
-{{ :university:courses:electronics:anv_f1.png?500 |}}
+{{ :university:courses:electronics:anr_f1.png?600 |}}
 <WRAP centeralign> Figure 1 Generating a negative voltage reference </WRAP>
-**Circuit Description**
+By examining figure 1(a) we see that, by the virtual ground nature of the inverting op amp configuration, the zener voltage +V<sub>REF</sub> is impressed across resistor R<sub>1</sub>. If R<sub>2</sub> is exactly equal to R<sub>1</sub> this same voltage V<sub>REF</sub> will also appear across R<sub>2</sub> but with the sign reversed with respect to ground. Since the voltage across R<sub>2</sub> is the same as that across the zener diode we can in effect replace R<sub>2</sub> with the diode in the feedback loop as in figure 1(b) and still produce the same voltage at -V<sub>REF</sub>. R<sub>Z</sub> simply sets the bias current level in the zener much as R<sub>Z</sub> in 1(a). In 1(b) I<sub>Z</sub> is equal to V<sub>DD</sub>/R<sub>Z</sub> where in 1(a) I<sub>Z</sub> is equal to (V<sub>DD</sub> - +V<sub>REF</sub>)/R<sub>Z</sub>. To design for the same I<sub>Z</sub> in both cases we simply change the value of R<sub>Z</sub>. Capacitor C<sub>1</sub> decouples the reference diode between its ground and output terminals. In addition low inductance 0.1 µF supply decoupling capacitors (not shown in the figure) are often connected to +V<sub>DD</sub> and -V<sub>SS</sub> very close to the op-amp.
-This circuit can use almost any three terminal voltage reference or regulator, and a low noise, low distortion, low offset operational amplifier. The Analog Parts Kit includes an ADP3300 3.3V voltage regulator and an AD584 voltage reference which we can use to build examples of this circuit.
+====Circuit Description====
-Notice in (b) that the reference is floating i.e. its ground terminal is not connected to the system ground, its input is still connected to the +V<sub>DD</sub> supply, its output is now connected to the inverting input of the op amp (through a 1 kΩ isolation resistor), and the GND pin is connected to the amplifier output. The circuit will not work if the GND pin is connected to the actual circuit ground. In this configuration, the reference block acts as a voltage source connected inside the feedback loop of the op amp. Negative feedback forces the op amp output to -V<sub>REF</sub> with respect to ground. The only errors in the output voltage are those due to the input offset voltage of the op amp and any error due to the reference itself. The error due to the bias current flowing through the 1 kΩ resistor is negligible since most modern op-amps have very low input bias current. The op amp used must, therefore, have low offset voltage and a rail-to-rail output is useful if the negative supply voltage is close to the negative reference output voltage.
-Headroom issues relating to the reference and the op amp must be considered in this circuit for proper operation. The V<sub>DD</sub> supply must be large enough so that the headroom requirement of the reference is met. Low drop out regulators can require a supply voltage headroom of as little as 300mV where other voltage reverences might require at least 1.5V (V<sub>IN</sub> - V<sub>OUT</sub>); therefore, +V<sub>DD</sub> should be at least 1.5 V higher than V<sub>OUT</sub>. In the case of the circuit in figure 1(b) V<sub>OUT</sub> is at 0V (ground potential) so +V<sub>DD</sub> need only be higher than ground by the required headroom. The requirement on the negative supply is determined by the op amp output stage headroom requirement. Some amplifiers have a rail-to-rail output stage; but, even so, at least several hundred millivolts output headroom should be allowed in this circuit. The OP482 amplifier supplied in the Analog Parts Kit is specified to typically need 1.1V of headroom on the negative supply for example which means the output should be able to swing to -3.9 volts when supplied by -5 volts.
+In theory this circuit can be built using almost any three terminal voltage reference circuit and a low noise, low offset operational amplifier. The lab activities on band-gap reference circuits [2] [3] [4] use NPN transistors to build positive voltage references. To build a negative reference based on the band-gap concept we would require high quality PNP transistors and the PNPs generally available in IC processes are not as high quality as the available NPN devices. These NPN based band-gap circuits will provide a couple of examples we can used to explore this negative reference configuration.
+The first circuit iteration in step 1 of this lab will use a diode as a reference and further iterations will substitute NPN transistor based two terminal ( shunt ) and three terminal ( series ) circuits as the reference element.
-Capacitor C<sub>1</sub> (0.1 µF) decouples the reference between its ground and output pins. The 1 kΩ resistor isolates the capacitor from the inverting input of the op amp. A low inductance 0.1 µF ceramic decoupling capacitor (not shown in the figure) should be connected to +V<sub>DD</sub> very close to the two ICs. In most cases, the final output of the op amp (-V<sub>REF</sub>) will be heavily decoupled, which means that the op amp used must be stable with large capacitive loads. A typical decoupling network consists of a 1 µF to 10 µF electrolytic capacitor in parallel with a 0.1 µF low inductance ceramic MLCC type.
+=====Materials:=====
+ADALM2000 Active Learning Module\\
-===== Materials: =====
-Analog Discovery Lab hardware\\
 Solder-less breadboard, and jumper wire kit\\
-1 kΩ resistor\\
+ - 4.7 KΩ resistor\\
- ADP3300 LDO positive voltage regulator\\
+- 1.5 KΩ resistors\\
- AD584 voltage reference\\
+- 20 KΩ resistors\\
-OP482 quad op-amp\\
+- 2.2 KΩ resistor\\
-0.01 uF Capacitor\\
+ - 100 Ω resistor\\
- 0.1uF Capacitor\\
+ - 10 KΩ variable resistor (potentiometer)\\
+- small signal NPN transistors (2N3904 and SSM2212)\\
+- LEDs (any color will do)\\
+ - OP482 or OP484 quad op-amp\\
+ - 1 nF Capacitor\\
+- 0.01 uF Capacitors\\
+- 0.1uF Capacitors ( supply decoupling capacitors for + and - 5 V supplies )
-===== Directions: =====
+=====Directions Step 1:=====
-First build the -3.3 volt circuit using the ADP3300 positive 3.3 volt regulator and one of the amplifiers in the OP482 quad op amp shown in figure 2 on your solder-less breadboard. Connect the positive 5 volt supply from the Discovery connector to +V<sub>DD</sub> and the negative 5V supply to -V<sub>SS</sub>. Use the two positive scope input channels to monitor the +V<sub>DD</sub>, -V<sub>SS</sub> and -V<sub>REF</sub> voltages. It is always good practice to ground the unused negative scope channel inputs when using the scope single ended. Be sure to connect the shutdown pin of the ADP3300 (pin 3 SD) to +V<sub>DD</sub>.
+The zener diode ( 1N4735 ) supplied in the Analog Parts Kit is a 6.1 volt diode. 6.1 volts is much too high a reverse breakdown voltage to build this circuit using the fixed +/- 5 volt power supplies of the Analog Discovery hardware. The forward voltage of an LED is in the range of 1.6 to 2.0 volts depending on the color of the diode. While not a proper reference diode, we can build the circuit for instructional purposes using the LEDs from the Analog Parts Kit.
-{{ :university:courses:electronics:anv_f2.png?400 |}}
+Build both of the versions of the circuits in figure 1(a) and 1(b) as shown in figure 2 on your solder-less breadboard. Use two LEDs preferably of the same color. Green LEDs will have a higher forward voltage drop than red or yellow. We want the diode current, I<sub>D</sub>, to be about 1 mA and the as close to this same value in both versions of the circuit. In the case (b) I<sub>D</sub>  will be +5/R<sub>4</sub> so a 4.7 KΩ resistor would give about 1 mA. In case (a) I<sub>D</sub> will be (+5-V<sub>D</sub>)/R<sub>3</sub>. If we use 2 V as an estimate for V<sub>D</sub>, then R<sub>3</sub> would be around 3 KΩ. You can get 3 KΩ by connecting two 1.5 KΩ resistors from the Parts Kit in series. Also for case (a) we need to pick values for R<sub>1</sub> and R<sub>2</sub>. We want the current in R<sub>1</sub> to be much smaller than the current in R<sub>3</sub>. So setting  R<sub>1</sub> and R<sub>2</sub> to a much higher value such as 20 KΩ should satisfy that condition.
-<WRAP centeralign> Figure 2 -3.3 volt regulator circuit </WRAP>
+{{ :university:courses:electronics:anr_f2.png?600 |}}
-===== Hardware setup: =====
+<WRAP centeralign> Figure 2, LED based volt regulator example </WRAP>
-Open the voltage source control and the voltmeter instrument windows from the main screen of the Waveforms software.
+=====Hardware setup:=====
-===== Procedure: =====
+Open the voltage source control and the voltmeter instrument windows from the main screen of the Waveforms software. A DMM, if available, could be useful to more accurately measure the DC voltages in the circuit than the Waveforms voltmeter instrument.
-Turn on both the positive and negative power supplies. Observe the voltage at -V<sub>REF</sub>, pin 14 of the op amp and at V<sub>OUT</sub> of the ADP3300 at pin 4.
+=====Procedure:=====
-===== Questions: =====
+Turn on both the positive and negative power supplies. Observe the two voltages at -V<sub>REF</sub>, pins 8 and 14 of the op amp and at +V<sub>REF</sub> on the LED.
-What voltage did you measure at -V<sub>REF</sub>? What voltage did you measure at pin 4 of the ADP3300? Are these the correct expected values and why?
+=====Questions:=====
-===== Directions: =====
+What voltage did you measure at -V<sub>REF</sub>for the circuits (a) and (b)? What voltage did you measure at the LED? Are these the correct expected values and why?
-Sometimes both positive and negative reference voltages are needed in a system. The AD584 voltage reference offers pin programmable selection of four output voltages: 10V, 7.5V, 5V and 2.5V. By selecting the 5V output option (connect pin 1 and pin 2 together) and using the 2.5V tap on the internal resistor string for the feedback point of the op amp we can split the 5V output into +2.5V and -2.5V with respect to ground.
+=====Directions Step 2:=====
-Now add the +2.5/-2.5 volt circuit to your solder-less breadboard using the AD584 positive voltage reference and a second amplifier in the OP482 quad op amp shown in figure 3. Be sure to turn off the power supplies before making any changes or additions to your breadboard.
+Modify your breadboard setup from step 1 as shown in figure 3. Be sure to turn off the power supplies before making any modifications to your breadboard. Replace the LED diode with the shunt regulator stage from earlier lab [3]. Resistors R<sub>1</sub>, R<sub>2</sub> and transistor Q<sub>1</sub> are connected as the zero gain amplifier from the earlier lab [5]. Resistor R<sub>3</sub> and transistor Q<sub>2</sub> are added as in the stabilized current source lab [6]. If the SSM2212 matched NPN pair is used it is best that it be used for devices Q<sub>1</sub> and Q<sub>2</sub>. Q<sub>3</sub>is added as common emitter stage, its base connected to the collector of Q<sub>2</sub> and collector connected to the combined node of R<sub>1</sub>, R<sub>3</sub> R<sub>4</sub>.
-{{ :university:courses:electronics:anv_f3.png?400 |}}
+{{ :university:courses:electronics:anr_f3.png?600 |}}
-<WRAP centeralign> Figure 3 +2.5V and -2.5V reference circuit </WRAP>
+<WRAP centeralign> Figure 3, NPN shunt band-gap reference example </WRAP>
-===== Hardware setup: =====
+=====Hardware setup:=====
 The setup is the same as step 1.
-===== Procedure: =====
+=====Procedure:=====
-Turn on both the positive and negative power supplies. Observe the voltage at -V<sub>REF</sub>, pin 8 of the op amp and at +V<sub>REF</sub> of the AD584 at pin 1. Also check to confirm that the 2.5V tap at pin 3 of the AD584 is at 0V.
+Turn on both the positive and negative power supplies. Observe the voltage at -V<sub>REF</sub>, pin 14 of the op amp and across the band-gap shunt regulator (collector and emitter of Q<sub>3</sub>) . You can adjust potentiometer R<sub>3</sub> to produce a -1.25V reference voltage.
-**Testing supply headroom**
+====Testing supply headroom====
-To test the headroom requirements for +V<sub>DD</sub>, disconnect the fixed positive power supply from +V<sub>DD</sub> and remove any supply decoupling capacitors. Be sure to turn off the power supplies before making any changes or additions to your breadboard. Now connect +V<sub>DD</sub> to AWG 1. Set AWG 1 to triangle waveform at 100 Hz. Set the amplitude to 2.5V with a 2.5V offset. Connect scope channel 1 to the output of AWG1 and connect scope channel 2 to -V<sub>REF</sub> of the first example circuit at pin 14 of the OP482. Use the oscilloscope instrument in the XY mode, scope channel for X and scope channel 2 for Y. Start AWG 1 and turn on the fixed negative 5V  power supply. Record the minimum +V<sub>DD</sub> voltage where -V<sub>REF</sub> starts to remain constant at -2.5V.
+To test the headroom requirements for +V<sub>DD</sub>, disconnect the fixed positive power supply from +V<sub>DD</sub> and remove any supply decoupling capacitors. Be sure to turn off the power supplies before making any changes or additions to your breadboard. Now connect +V<sub>DD</sub> to AWG 1. Set AWG 1 to trapezium (trapezoid)  waveform at 100 Hz. Set the amplitude to 2.5V with a 2.5V offset for a 0 to +5V swing. Connect scope channel 1 to the output of AWG1 and connect scope channel 2 to -V<sub>REF</sub> of the first example circuit at pin 14 of the OP482. Use the oscilloscope instrument in the XY mode, scope channel for X and scope channel 2 for Y. Start AWG 1 and turn on the fixed negative 5V  power supply. Record the minimum +V<sub>DD</sub> voltage where -V<sub>REF</sub> starts to remain constant at -1.25V.
-Move scope channel 2 to the +V<sub>REF</sub>, pin 1 of the AD584, and -V<sub>REF</sub>. pin 8 of the OP482, and again record the minimum +V<sub>DD</sub> where both +V<sub>REF</sub> and -V<sub>REF</sub> start to remain constant.
+To test the headroom requirements for -V<sub>SS</sub>, reconnect +V<sub>DD</sub> to the fixed positive power supply. Disconnect the fixed negative power supply from -V<sub>SS</sub> and remove any supply decoupling capacitors. Now connect -V<sub>SS</sub> to AWG 1. Set the amplitude to 2.5V with a -2.5V offset for a 0 to -5V swing. Start AWG 1 and turn on the fixed positive 5V  power supply. Repeat your measurements of pins 14 of the OP482 recording the lowest value for -V<sub>SS</sub> where the reference voltage is constant.
-To test the headroom requirements for -V<sub>SS</sub>, reconnect +V<sub>DD</sub> to the fixed positive power supply. Disconnect the fixed negative power supply from -V<sub>SS</sub> and remove any supply decoupling capacitors. Now connect -V<sub>SS</sub> to AWG 1. Set the amplitude to 2.5V with a -2.5V offset. Start AWG 1 and turn on the fixed positive 5V  power supply. Repeat your measurements of pins 14 and 8 of the OP482 and pin 1 of the AD584 recording the lowest value for -V<sub>SS</sub> where the reference voltages are constant.
+=====Questions:=====
-===== Questions: =====
-The AD584 uses an internal resistor divider. What makes these resistors any better than using resistors from your parts kit? (Hint: the answer is in the AD584 datasheet)
+=====Directions Step 3:=====
-How do the headroom measurements you made compare to the values specified in the ADP3300, AD584 and OP482 datasheets.
+Modify your breadboard setup from step 1 as shown in figure 4. Be sure to turn off the power supplies before making any modifications to your breadboard. Change the two terminal, shunt, regulator used in step 2 to the three terminal reference [2] by adding emitter follower stage Q<sub>4</sub>, and compensation capacitor C<sub>1</sub>.
+{{ :university:courses:electronics:anr_f4.png?600 |}}
+<WRAP centeralign> Figure 4, NPN three terminal band-gap reference example </WRAP>
+=====Hardware setup:=====
+The setup is the same as step 1.
-==== For further reading: ====
+=====Procedure:=====
-[[http://www.analog.com/static/imported-files/data_sheets/ADP3300.pdf|ADP3300 datasheet]]
+Turn on both the positive and negative power supplies. Observe the voltage at -V<sub>REF</sub>, pin 14 of the op amp and across the band-gap three terminal regulator (emitter of Q<sub>4</sub> and emitter of Q<sub>3</sub>).
-[[http://www.analog.com/static/imported-files/data_sheets/AD584.pdf|AD584 datasheet]]
-[[http://www.analog.com/static/imported-files/data_sheets/OP282_OP482.pdf|OP482 datasheet]]
-**Return to Lab Activity [[university:courses:electronics:labs|Table of Contents]]**
+Repeat the supply headroom tests you did in Step 2 for this configuration. Are there any differences?
-==== Appendix: ====
+====For further reading:====
-As was pointed out, this technique can be used with almost any "three" terminal voltage reference. In figure 4 we show the REF43 positive 2.5 volt reference being used to generate a negative 2.5V reference.
+[1] [[university:courses:electronics:electronics-lab-26|Activity: Zener Diode Regulator]], [[university:courses:eps:zener-regulator|EPS Activity: Zener Diode Regulator]]\\
+[2] [[university:courses:electronics:electronics-lab-9|Activity 9.  Regulated Voltage Reference]]\\
+[3] [[university:courses:electronics:electronics-lab-10|Activity 10. Shunt regulator]]\\
+[4] [[university:courses:eps:band-gap-regulator|EPS Activity: The Band-Gap Voltage Reference]]\\
+[5] [[university:courses:electronics:electronics-lab-7|Activity 7. Zero gain amplifier (BJT)]]\\
+[6] [[university:courses:electronics:electronics-lab-8|Activity 8. Stabilized current source (BJT)]]\\
+[[http://www.analog.com/static/imported-files/data_sheets/OP282_OP482.pdf]] OP482 datasheet
-{{ :university:courses:electronics:anv_f4.png?400 |}}
+Return to Lab Activity [[university:courses:electronics:labs|Table of Contents]]
-<WRAP centeralign> Figure 4 -2,5V reference using a REF43 </WRAP>
-[[http://www.analog.com/static/imported-files/data_sheets/REF43.pdf|REF43 datasheet]]
+====Appendix:====

Wiki

Analog Devices Wiki

Differences

Analog Devices Wiki

Differences

Page Tools