All instruments have a chain of operation from input to output. A physical variable, such as temperature, pressure, light, or position, is picked up by a sensor, converted into an electrical signal, passed through one or more stages of electronics, and presented as a numeric or visual output. Knowing the whole chain is much easier to understand than looking at individual components that have no relationship to each other.
If you take a piece of paper, you could draw a series of five blocks and connect them with arrows. Start from the left side and call the first block physical input, then sensor, followed by signal conditioning, followed by conversion, and end with display. Let’s say that you want to measure the room temperature. The physical input would be the air temperature. The sensor would be a temperature probe which responds to heat. The signal conditioning circuit would be a small circuit which converts the sensor’s response into a measurable voltage. The converter would be a device like an ADC which converts the analog voltage into digital values. Finally, the display is just a screen showing the number. Now every component in the chain has a specific function and we don’t need to know how it works, just what it does.
The sensor is the point at which the instrument begins to respond to the measurand. In some instruments, the output of the sensor will be a change in resistance, in others a change in voltage, current, capacitance, or frequency of pulses. That output might not be strong enough, or in a usable form to be applied directly to the rest of the instrument. If the sensor is a thermistor, its resistance will change with temperature, but it can’t be connected directly to a meter. It must be used as part of a voltage divider which converts the change in resistance into a change in voltage. The voltage divider output is a voltage that can be measured with a multimeter or used to drive other circuits.
The signal conditioning stage is the part of the instrument which is located between the sensor and the next stage. It may contain an amplifier, which increases the amplitude of the sensor output, or a filter which removes spurious signals. Other types of signal conditioning circuits may offset the signal, so that a zero input results in a zero output, or limit the signal range, so that it is suitable for an ADC. When drawing the block diagram, omitting this stage can lead to confusion, because the voltage at the final stage of the circuit may differ greatly from the voltage at the sensor.
Newcomers to the subject of electronics tend to follow a single visible wire all the way through a circuit and assume that the signal on that wire is the final measured value. In reality, that wire may go through a plug, a few resistors, an amplifier, a filter, a converter, before it reaches the display. In addition to the signal wire, you should also note where the power supply is connected, as well as the ground, the inputs and outputs of the various blocks. Just because a signal path has been traced from beginning to end doesn’t mean that the stage is working properly if the power supply or ground is disconnected.
When a reading is incorrect, the block diagram can serve as a troubleshooting aid. Check that the sensor is receiving the correct physical input, and that its supply voltage and output are within normal limits. Then move on to the signal conditioning stage. Is the input to that stage correct? Is the output? Then move to the next stage, and then the next, until you reach the display. Don’t change more than one thing at a time. Record your measurements, units, ranges, and test conditions. You’ll find out where the signal ceases to behave normally.
With a properly drawn block diagram, you should be able to look at any stage and explain what goes in, what comes out, and why the change occurs. Next time you draw a diagram, look for the first stage where the signal undergoes a transformation, not just a change in magnitude. It may reveal the function of the stage better than the name of the component.