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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 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, R1 and R2. Errors in the matching, in addition to any offset voltage in the op amp, will produce errors at the negative output -VREF. 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.
Figure 1 Generating a negative voltage reference
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. 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 +VDD 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 -VREF 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 VDD 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 (VIN - VOUT); therefore, +VDD should be at least 1.5 V higher than VOUT. In the case of the circuit in figure 1(b) VOUT is at 0V (ground potential) so +VDD 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.
Capacitor C1 (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 +VDD very close to the two ICs. In most cases, the final output of the op amp (-VREF) 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.
Analog Discovery Lab hardware
Solder-less breadboard, and jumper wire kit
1 1 kΩ resistor
1 ADP3300 LDO positive voltage regulator
1 AD584 voltage reference
1 OP482 quad op-amp
1 0.01 uF Capacitor
1 0.1uF Capacitor
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 +VDD and the negative 5V supply to -VSS. Use the two positive scope input channels to monitor the +VDD, -VSS and -VREF 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 +VDD.
Figure 2 -3.3 volt regulator circuit
Open the voltage source control and the voltmeter instrument windows from the main screen of the Waveforms software.
Turn on both the positive and negative power supplies. Observe the voltage at -VREF, pin 14 of the op amp and at VOUT of the ADP3300 at pin 4.
What voltage did you measure at -VREF? What voltage did you measure at pin 4 of the ADP3300? 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.
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.
Figure 3 +2.5V and -2.5V reference circuit
The setup is the same as step 1.
Turn on both the positive and negative power supplies. Observe the voltage at -VREF, pin 8 of the op amp and at +VREF of the AD584 at pin 1. Also check to confirm that the 2.5V tap at pin 3 of the AD584 is at 0V.
Testing supply headroom
To test the headroom requirements for +VDD, disconnect the fixed positive power supply from +VDD 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 +VDD 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 -VREF 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 +VDD voltage where -VREF starts to remain constant at -2.5V.
Move scope channel 2 to the +VREF, pin 1 of the AD584, and -VREF. pin 8 of the OP482, and again record the minimum +VDD where both +VREF and -VREF start to remain constant.
To test the headroom requirements for -VSS, reconnect +VDD to the fixed positive power supply. Disconnect the fixed negative power supply from -VSS and remove any supply decoupling capacitors. Now connect -VSS 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 -VSS where the reference voltages are constant.
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) How do the headroom measurements you made compare to the values specified in the ADP3300, AD584 and OP482 datasheets.
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.
Figure 4 -2,5V reference using a REF43