Opamp-Circuits (Page 4)

Input Impedance of this module is Ri as pin 2 is at virtual ground, the opamp with feedback tries to maintain pin 2 and 3 at same potential pin 3 is at 0V hence pin 2 is at virtual ground. Clamping diodes protect OpAmp, Rf + Ri is between 5kE and 1ME as an opamp may be able to drive around say 5mA max.

Inverting Amplifier – Op-Amp Circuits

Current into node pin 2 = Vin/Ri if Vin is +ve it raises potential at pin 2, in order to bring it to 0V the OpAmp sucks away the current by turning its output negative the current leaving pin 2 node is also Vin/Ri. Then Vout is given by Vin/Ri * Rf as per V=IR ohms law. Most OpAmps output swings around 1v less than VCC/VDD for full swing use CA3130 this is a FET input OpAmp, and has low bias currents in pico amps.
Inverting Amplifier - Op-Amp Circuits

Vout = Vin * (-1) * (Rf/Ri)

Precision Amplifier with Digital Control

This is a modification of a mV Source that can be whipped up easily. You could use a DPM or Multimeter to read the output. The ability of this circuit to perform well depends on the quality of all the MFR resistors and the MultiTurn Pot. Use a Bourns 10T Pot.

Good Soldered Joints, Keep all Resistors and temperature sensitive parts from Transformer and Regulators. Keep Ripple in power supplies low, no EMI tolerated. If you have problems, make a Battery Powered Unit. Shield well in case you are in a Electrically Noisy environment.

Millivolt Source In this link see at bottom this circuit millivolt source, pdf.

I have put a better offset null, OP07 has around 75uV offset error which may show as +/- 1 count error on 4 1/2 DPM 19999 counts. You can skip it if you are using a 3 1/2 digit DPM as the error will not show, even it 4 1/2 it may be upto 2 counts only.

R9, P4 and R10 are for balance and offset as you said you can use it that way. (old circuit)

C7 can be a low leakage plastic cap, even a tantalum electrolytic is ok, aluminum electrolytic may cause a very small error.

Q1 can be any npn that can take 100mA current, do not use RF devices, 2N2222 is best.

If you use a DPM protect DPM inputs with clamping diodes or zeners or an error in bread-boarding may send +/- 12V to DPM and it may be damaged. Some DPMs come with protection like DMMs. use the circuit in del2003.pdf in analog section to make a 4 1/2 DPM.

Also in 2000mV range do not short outputs as the Q1 may get damaged, and in 200mV and 20mV range the output impedance is 10 ohms which is good for calibrating any high input impedance instrumentation like a process indicator etc. loading with 100K 10K will cause error. Most instruments are very high impedance so it is fine.

This is a LED Analog Meter, This can be used as a Resistance Meter and Low Impedance Voltmeter for Battery Levels. To measure battery voltage, the R5-R12-R17 etc. part of the Reference Resistor Divider Network can be modified to suit. Shown here is for 4 LEDs, Use Three LM324 for 12 or More LEDs and Cascade as shown.

Resistance Measurement Analog LED Meter

This cannot Measure Voltage levels from High Impedance Sources, will work for Battery Voltage Tests. To make it into a Continuity tester. R27 must be a short and R23 5 Ohms. The Black probe should have a Built in Resistance of 2 Ohms. If you want it to be a dedicated voltmeter, remove R3, The Probe has to be a 10X Attenuator with 10M Ohm and The Resistor Divider Steps in 100mV per Step. The R27, R23 etc. is 20K. A Leakage Tester a Mains Voltage Monitor are other possibilities. Use LM3914 for a easier solution. A nice book for your Design Library – Measuring Circuits By Rudolf F. Graf

This is easy to rig millivolt source for field calibration or troubleshooting of 4-20 mA current loops. Here a Darlington pair is used for current amplification which reduces the Ib error as gain is very high.

Millivolt Source - Field Callibration Current Loop

A rotary switch selects, 4-12-20 mA Preset points. A Bourns multi-turn wirewound Pot can also be used with a digital dial. Enclose in a dust proof handheld box. Read more on process calibration.

When a Inverting Opamp Configuration is at a steady state, we say the Inverting Input is at a Virtual Ground. That means it is at 0V w.r.t to the dual power supply ground, but it cannot drive or draw any current. It is at a high impedance, but still at 0V. When you buffer this 0 V, you get a low signal ground for a opamp supply.

Opamp Supply on Buffered Virtual Ground

This gnd. can sink and source in a couple of mA. You can use it with low power opamp circuits for portable battery operated devices. This creates a virtual +/- 6 V dual supply from a 12V battery. This may be needed in cases where some instrumentation opamps need the negative supply or your design demands a measurement around zero. You may get a more loadable ground using a Power Opamp, i have not tried. The above circuit gnd cannot be used as a return path for LED’s or Relays. You can drive these, between VCC-VDD, but translate levels to drive them.

In this circuit we tackle the error indicated in the earlier Current Source. The LM336-2.5V eliminates the tiny error of the regulated supply and resistors. Thereby increasing Precision to a higher degree.

Precision Op-Amp Current Source

The opamp mirrors the stable 2.5V across P3 + R13. With P3 Bourns 10 Turn Trimpot you can trim the current for calibration. Q1 BC557B having a Beta – hfe of 200 is used. But a higher gain or a FET here may reduce error further, that may be needed if you are going for 16 Bit or more resolution. Then even opamp needs to change.

Suppose you build it with the best Opamp, FET etc., but place it close to a Warm transformer, Regulator chip or even a Cooling Fan, you will see the lower digits of a 5-1/2 DMM spinning fast.

Here is a current source you can build for resistance measurement. When the current is held constant, you know as per Ohm’s Law the Voltage across Resistor is proportional to Resistance value.

Precision Current Source for Resistance Measurement

The supply is +12 and -12, The total voltage across R6 + R7 is 24V. Then 24V / 120K = 0.2mA. The voltage across R6 is (10K * 0.2mA) = 2V. The same is reflected across R5 in this feedback configuration. That means Q3 is a 2V / 1K = 2mA source. If my calculations are right.

There are sources of errors in this circuit. The temperature variation of all resistor values, which is 100ppm for general calculations in 1% MFR. Let us assume you use OP07 which is close to an ideal opamp, but for this application it is not needed. The second error is Ib, the base current of Q3 which may be 0.2mA / Hfe(200) = ~ 1 uA. Then the variation of Hfe, Vcc and Vdd w.r.t. Temperature, should not be overlooked. Use LM7812 and LM7912.

So you see, design knowing that all these components are not ideal. Leakage currents, Humidity, EMI, Stray Capacitance and Inductance and much more. It is just like, even when the motor is fixed firmly on the machine, some parts Vibrate and create a Noise due to Mechanical Resonance. So Build and evaluate your design in the real environment, to learn.

Discover how resistors are color coded – Interactive Java Resistors Tutorial.

Studying current measurement is a prerequisite for many of the measuring techniques. The current parameter mainly specifies the power consumption in a circuit, given the value of resistance. It is found convenient to measure current rather than voltage for knowing power output and determining efficiency. It may be required to measure leakages in circuits at certain times. Hence the measurement of current constitutes a priority.

Ammeter and Precision Rectifier

Measurement of DC Current –

The circuit diagram for the measurement of current (d.c. and a.c. modes) is shown aside. For measurement of current switch SI is operated. The switch S-ad is kept in d.c. mode. This enables the current to pass through a shunt circuit consisting of resistors R26, R27, R28, R29 and R 30. The current ranges are provided in 5 decades i.e. 200 micro-amps, 2 milli-amps, 20 milli-amps, 200 milli-amps and 2 amps. An additional current range that can be read upto 20 Amps is also provided. However, for measuring this high current the green terminal provided on the meter should be used. When a current to be measured is fed to the input terminals of the instrument appropriately, a voltage proportional to the current through the shunt resistor is fed into the DPM which measures the d.c. voltage which in turn indicates the d.c. current being fed.

Measurement of AC Current –

In case of a.c. measurement, the switch S-ad is kept in a.c. mode. The a.c. current path is similar to the d.c. current path in the shunt resistor. However the voltage tapped across the shunt resistor is fed into IC2 which is a buffer. The output of IC2 is fed to IC3 through capacitors C10 and C11. This IC is an operational amplifier acting as a precision rectifier. The output of IC3 is fed to the input of the DPM for measuring the a.c. current being fed to the input terminals. It can be seen that the current measurement is similar to the voltage measurement except that the attenuator chain is replaced by the shunt resistor circuit.

(This is scanned-ocr from my earlier file, some mistakes corrected – delabs)