Lab 6
Construct an Analog Nanoammeter

In this exercise you will construct an Analog Nanoammeter by adding components to a printed circuit board. You will learn how to prepare the printed circuit board for the components and how to solder components to the board.

Reading Assignment

Building Scientific Apparatus. Section 1.3.1 (Soldering). Section 6.10.3 (Printed Circuit Boards).

Supplies and Materials

Student Workstation. Card Edge Connector. Printed circuit board. Operational amplifier: OP97FP. Resistors: 1/4 W: 470 Ω, 10 kΩ (2), 100 kΩ, 4.99 MΩ 1% (2). Potentiometer, 20 turn trim: 100 kΩ. Capacitors: 0.022 µF, 0.1 µF (2). Diode, 1N914B (2). Hookup wire, tinned copper (AWG 22). Wire cutters. DIP socket (8 pin), DigiKey AE7208. Digital multimeter (4-1/2 digit). Soldering Iron; 60-40 Solder. Soder-Wick (Solder Removal Company, Covina, CA). Razor blade, single-edged; Pliers (needle-nose).

I.    Introduction

A nanoammeter is an inverting amplifier with a feedback resistor, Rf, selected so that an input current of 1 nA will produce an output voltage of 5 mV. The value of the feedback resistor can be calculated from the gain of the amplifier. See Figure 1.


Figure 1. Printed Circuit Board Assembly


II.    Assemble the Printed Circuit Board

Assemble the printed circuit board using the circuit diagram and a photograph of the final board assembly as a guide. See Figure 2.


Figure 2. Printed Circuit Board Assembly

Solder all the components to the board using the procedure demonstrated by your instructor and described in your text. The component leads should protrude about 1/16" from the back of the board. Work slowly and carefully. Avoid cold solder joints. Use Soder-Wick or a razor blade to remove excess solder and solder bridges using the procedure demonstrated by your instructor.

Position the op-amp in its socket after all the components have been soldered to the board. Insure that the op-amp is oriented correctly. It is good practice to avoid touching the pins of the op-amp unless you are properly grounded. Ask your instructor how to provide a proper ground.

III.    Prepare for Calibration

Insert your nanoammeter into the socket on the Card Edge Connector. The nanoammeter components must face the row of pins on the Card Edge Connector. Turn the DC power supply ON or OFF by plugging its AC power cord into or out of a wall receptacle. If the card edge connector is not mounted on your breadboard see the Card Edge Connector instructions on the resources page.

Do not insert or remove your nanoammeter from the Card Edge Connector if the
power supply is turned ON.

Construct a simple current source to measure the op-amp output of your nanoammeter. Use the -12 V output from the breadboard power supply as the source of a voltage and a voltage divider to adjust the voltage, –V, across a 4.99 MΩ resistor. The voltage divider consists of a 100 KΩ resistor in series with a 100 KΩ potentiometer. The 4.99 MΩ resistor is connected to the moveable contact (wiper) of the potentiometer. The input of the nanoammeter is connected to this resistor and one end of the voltage divider to the ground connection (GND) of the nanoammeter. See Figure 3.


Figure 3. A Simple Current Source

Turn the DC power supply ON. Copy the table from Figure 4 into your lab notebook. For each value of input current to your nanoammeter shown in the first column calculate (from Ohm’s Law) the voltage, –V,  that must be applied across the 4.99 MΩ resistor in the current source. Enter the voltage in the second column of the table.


Figure 4. Calibrate the Nanoammeter

V.    Calibrate the Nanoammeter

Adjust the voltage divider in the current source so that the calculated voltage appears across the resistor. Measure the output of the op-amp. Enter this voltage in the last column of the table.

If you cannot adjust the voltage to zero, disconnect the resistor and use a short wire to connect the nanoammeter input directly to ground (GND).

Turn the DC power supply OFF. Discuss the accuracy of the measurements entered above in your notebook. Consider the tolerance of the 4.99 MΩ resistor and the accuracy of reading the divider output.

  1. VI.Perform a Least Squares Analysis

A negative current supplied to the input of the op-amp will produce a positive voltage at its output. A calibration procedure will determine the functional relation between them from a linear Least-Squares analysis. An op-amp is an inherently linear device. As a result, the input current of the op-amp, I, and its output voltage, Vop, will be related by an equation of the form:

  1. I = mVOP + b

where, m, is the slope and, b, is the intercept:

m = [ n ∑ xi yi - ( ∑ xi ) ( ∑ yi ) ] / [ n ∑ xi2 - ( ∑ xi )2 ]

b = [ ( ∑ yi ) ( ∑ xi2 ) - ( ∑ xi yi ) ( ∑ xi ) ] / [ n ∑ xi2 - ( ∑ xi )2 ]

Here, n, is the number of data points; xi is the ith value of the current, and yi is the corresponding output of the op-amp. Record the slope and intercept in your lab notebook.

Copy the graph from Figure 5 in your lab notebook. Plot your data and the least squares line.


Figure 5. Least Squares Analysis

VII.    Homework Assignment

Update your lab notebook to include this exercise and questions raised in class.

Read the next laboratory exercise.

Complete GTutorial Exercise 8 and Exercise 9.