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Techniques for ensuring output/input impedances are compatible


Guest bbaker
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Hi. I am new to electronics engineering and have a basic question...
I am chaining an RMS-DC Converter (AD8436) and an A/D Converter (AD7788) and the A/D does not have an input buffer. The AD7789 data sheet warns that DC gain errors can be introduced depending upon the output impedance of the source signal (page 17).
I plan on using the output amplifier of the AD8436 so I am sure it will be fine - but that is a gut feeling. How do you "design" your solution to make sure that the impedances are compatible? What electrical characteristics am I looking for - and if they are incompatible how should I address the problem?
Thanks,
Bryon

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You're right. It has a built-in input and output buffer. If you design the circuit in accordance with the datasheet, you won't have any problems.

What are you doing with the data once it's been sampled by the ADC? Why not bais the ADC input at half the supply voltage or use a precision full wave rectifier and calculate the RMS value using a microcontroller?

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Hi,
Thanks for that - I figured it would be okay. Can you give me any guidance on the basics of ensuring electrical compatibility between devices? What do you look for, and what techniques are commonly used to address incompatibility?

Re using the CPU to calculate the RMS - yes that is possible but the sample rate required to do that accurately is far beyond the processing capability of the CPU (and the chosen CPU is a constraint in the design). But if we consider that hypothetically for a moment it illustrates my main question quite well.... If using a Current Transformer to measure RMS with a 100 Ohm burden resistor, what would I have to do to make sure I could feed that signal directly into the A/D converter without introducing errors?

Thanks!

Bryon

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