Wednesday, December 6, 2023

Low Cost High Impedance Buffers for Oscilloscopes and Voltmeters

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Most of the low cost digital oscilloscopes and voltmeters have input resistance around 1 MOhm and input capacitance around 100pF. But in many case we are in need of input resistance above 10 MOhm and input capacitance of below 20pF. That is the case when we wish to see and measure signal from high impedance buffers as filters and some sensors. Most of the signals in the electronic circuits are in the range of +-15V and can be captured with popular operational amplifiers.

Modern op-amps can drive loads of 2 kOhm and that is enough for most digital and analog oscilloscopes and voltmeters. Also we are in need to capture differential signals with low voltage levels. That can be the case of components in the electronic circuits which are not connected to the ground.

For that purpose we are in need of high impedance buffers which convert the differential signals into normal signals with reference to the ground. The industry offers some electronic buffers, pre-amplifiers and active probes with high impedance for oscilloscopes and voltmeters. But they are relatively easy to be damaged and are expensive to be repaired or replaced. They are not suitable for the hobbyist and many educational institutions in the field of electronics.

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That paper is describing two simple and low cost high impedance buffers which solve the problems described above. The bandwidth of the buffers can be more than 1 MHz and depends mainly on the operational amplifier. Also during the practical exercises with students the equipment is frequently damaged due to inappropriate usage. The proposed buffers works also as protection blocks for the expensive oscilloscopes and voltmeters.

Buffer with four channels

Figure 1. Schematic diagram of the buffer with four channels and with unity gain for each channel. The buffer is used mainly for multi-meters and oscilloscopes.

Figure 1 : presents the schematic diagram of the high impedance buffers with four channels and with unity gain for each channel. The buffer is used mainly for multi-meters and oscilloscopes. Each channel of the pre-amplifier has unity gain.

The input signals are applied to the connectors CON1, CON2, CON3 and CON4.
The input resistance of each channel is 20MOhm.
The switches S1, S2, S3 and S4 can reduce the input voltage 10 times if needed
The diodes D1 to D16 protect the inputs of the operational amplifiers.
The pre-amplifier is build around quadruple single op-amp.

This makes the circuit with lowest price and with possible smallest size. Some of the op-amps appropriate for the buffer are listed in Table 1.

Operational amplifier Bandwidth,
Offset voltage, mV,
(typical – maximal)
Standby current, mA, (typ – max) Typical slew rate,
TL074 4 3.0 – 10 18 5.6-10mA 13
TL084 4 5.0 – 15 25 5.6-11.2mA 13
TL064 2 3.0 – 15 47 0.8-1.0mA 6

Table 1. Some operational amplifiers appropriate for the buffers in the article


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