Amplifier Circuit Diagrams

Meter Adapter with Symmetrical Input Schematic Circuit Diagram

Addressing Grounding Challenges in Oscilloscope Measurements

Ordinary volt meter differ significantly from oscilloscopes, especially in terms of their grounding setup. In most cases, an oscilloscope’s input has one side (GND) directly connected to the ground via the mains lead. However, this configuration can pose significant problems in specific scenarios. If the measuring probe is linked to a circuit that is also grounded, there’s a risk of introducing a short circuit within the circuit. Beyond affecting the circuit and the measurement, this situation could lead to severe consequences. Especially when dealing with high current or high voltage circuits, such as valve equipment.

Fortunately, overcoming this challenge is relatively straightforward. All that’s required is to ensure the oscilloscope’s input is isolated from ground. The instrumentation amplifier presented here achieves this while also functioning as an attenuator. The AD621 from Analog Devices amplifies the input by a factor of 10, and a switch at the input offers a choice of three ranges. Additionally, a ‘GND’ position has been incorporated to calibrate the oscilloscope’s zero setting.

Meter Adapter with Symmetrical Input Schematic Circuit Diagram

Ensuring Safety and Compatibility with Maximum Input Voltage

When operating this circuit, it’s crucial to adhere to the maximum input voltage limit, which should never exceed 600 VAC. To guarantee the functionality of R1 and R8, both resistors must possess a working voltage of at least 600 V. An effective approach is to use two identical resistors connected in series, especially since 300 V types are readily available. Additionally, it’s essential to ensure that all resistors employed have a tolerance of 1% or better to maintain precision in the circuit’s performance.

Additional Specifications and Suitable Alternatives

The AD621 boasts specific technical specifications. With an amplification factor of 10, it achieves a CMRR of 110 dB and operates within a bandwidth of 800 kHz. In case the AD621 is unavailable locally, a viable alternative is the AD620. However, it’s important to note that the bandwidth is limited to approximately 120 kHz with the AD620.

Versatile Power Options and Enhanced Efficiency

This circuit offers versatility regarding power sources. While it can be enclosed within a metal case powered by a mains supply, it performs exceptionally well when powered by two 9 V batteries. Moreover, the circuit demonstrates efficient power management, consuming only a few milliamps. For further reduction in power consumption, one can consider increasing the value of R9 to 10 k, optimizing the circuit’s energy efficiency.


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