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ADS1115 module and Voltage divider
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Please be positive and constructive with your questions and comments.

ADS1115 module and Voltage divider

by Sandspark2 on Wed Nov 17, 2021 9:01 am

Good morning everyone,

Should I use a filter or a voltage follower to measure the voltage of the voltage divider circuit in ADS1115?

For example, Arduino's Analog Input is a high impedance, so connecting the voltage divider circuit directly is not a problem (maybe a bypass capacitor is needed).
However, if you are using, for example, an ADS1115 module, I think you might not want to use it in the same way. However, if you google it, you can find many examples of measuring voltage by connecting directly to A 0~3 pins.

I would like to hear the opinions of those who understand the characteristics of the module and ADC well.

Sincerely yours,

Sandspark2
 
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Re: ADS1115 module and Voltage divider

by adafruit_support_mike on Wed Nov 17, 2021 7:24 pm

The ADS1115 is an ADC: it reads an analog voltage on its input pin, and converts that to a digital code.

The ADS1115 has a switched capacitor sample-and-hold circuit at its input that acts like a resistor whose value depends on the sampling frequency. The effective resistance is between 3M and 100M, which is high enough that most input signals don't need an op amp voltage buffer.

Your question about a voltage divider seems to be related to digital potentiometers like the DS1841:

https://www.adafruit.com/product/4570

For those, you generally want to use the output resistance without an op amp buffer. If you need/want a buffer, it would be better to use a DAC with one already built in.

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Re: ADS1115 module and Voltage divider

by Sandspark2 on Wed Nov 17, 2021 7:55 pm

Thanks for the reply!

So you are saying that the ADS1115 has a high enough impedance that there is no need to use a buffer?
For example, when using the link below, don't I need to take into account the shunt current flowing into the ADS1115?

https://www.researchgate.net/publication/342702365_Voltmeter_Design_Based_on_ADS1115_and_Arduino_Uno_for_DC_Resistivity_Measurement

https://www.spiria.com/en/blog/iot-m2m-embedded-solutions/measuring-small-currents-adc/

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Re: ADS1115 module and Voltage divider

by sj_remington on Wed Nov 17, 2021 8:09 pm

don't I need to take into account the shunt current flowing into the ADS1115?

In almost all cases, it is safe to assume that NO current flows into the ADS1115 analog inputs.
As already stated, in the effective input impedance approaches 100 Megohms.

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Re: ADS1115 module and Voltage divider

by Sandspark2 on Wed Nov 17, 2021 9:10 pm

In almost all cases, it is safe to assume that NO current flows into the ADS1115 analog inputs.

I see it is made to be very easy to use.
Is there an exception, for example, when the output impedance of the voltage divider circuit is high?

Sandspark2
 
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Re: ADS1115 module and Voltage divider

by sj_remington on Wed Nov 17, 2021 9:50 pm

If the voltage divider has high impedance, you should expect an error due to loading.

One usually doesn't know the exact input impedance of the ADC, but the introduced error can be estimated from the typical values, which range from 3 to 100 Megohms, depending on the FS selection (see the ADS1115 data sheet).

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Re: ADS1115 module and Voltage divider

by adafruit_support_mike on Wed Nov 17, 2021 11:54 pm


Both of those articles talk about measuring high resistances and/or low currents. In those cases, you have to consider the amount of voltage drop you'll see across the signal source due to the ADS1115's input current.

An op amp buffer doesn't automatically solve that problem though. In exactly the same cases, you have to be careful about an op amp's input offset current. The old reliable LM358's input offset current can be as high as 250nA, which will produce 2.5mV across a 10k resistor.

If you want to use an ADS1115 in a 5V circuit with less than 1LSB of error due to the ADS1115's input resistance, you can only afford 35uV of loading offset. 5V/3M=1.7uA, and 35uV/1.7uA=20.6 Ohms. So among other things, your analog input to the ADS1115 needs to have less than 20 Ohms of resistance.

For that to be useful, you also have to guarantee less than 35uV of combined error from all other sources.. the power has to be regulated to within a few microvolts, you have to avoid or compensate for thermocouple voltages across every connector[1] and solder joint[2], and you even have to watch out for airflow[3].

If you want a good reference on the subject, take a look at Keithley's _Low Level Measurements Handbook_:

https://www.tek.com/llm-handbook

When you get below 1mV, nothing is trivial.

[1] - Most connectors generate about 500nV/C, but can go as high as 2uV/C.

[2] - Copper wires from different manufacturers can generate 200nV/C. Copper-to-solder generates 1uV/C to 3uV/C. Copper to Kovar (most device leads) generates 40uV/C. Copper to copper-oxide (dirty connections) can generate 1mV/C.

[3] - You can produce 10nA to 50nA of current by pointing a fan at a circuit. The airflow literally blows electrons from one place to another.

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Re: ADS1115 module and Voltage divider

by Sandspark2 on Thu Nov 18, 2021 1:38 am

Thanks, both of you! There are so many factors to take into account.

One usually doesn't know the exact input impedance of the ADC, but the introduced error can be estimated from the typical values, which range from 3 to 100 Megohms, depending on the FS selection (see the ADS1115 data sheet).


On a related note, I have one more question about ADS1115 itself.
You say that the input impedance is above the several MΩ, but the datasheet says that changing the Gain will dramatically change that value, is this understanding correct?

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Re: ADS1115 module and Voltage divider

by adafruit_support_mike on Thu Dec 02, 2021 3:57 pm

Yes.

The circuit is a switched capacitor, and those are roughly equivalent to resistors whose value is proportional to the switching frequency.

The mechanical analogy is moving water from one bucket to another with a cup. Each cupful moves the same amount of water, and the amount of water you move per minute depends on how many times you fill and empty the cup per minute. The adjustable parameters are the volume of the cup and the fill/empty frequency. You can double the transfer rate by making the cup twice as big and dipping at the same frequency, or by dipping the same cup twice as fast.

Electrically, the charge necessary to put a given voltage between a capacitor’s plates is equivalent to the water, letting the capacitor charge to an input voltage is equivalent to filling the cup from one bucket, and discharging the capacitor to a fixed reference voltage is equivalent to emptying the cup.

You can’t control the size of the ADS1115’s sampling capacitor, but you can change the charge/discharge frequency. Lower frequencies move less charge per second, so they make the equivalent resistance higher. Higher frequencies move more charge per second, so they make the equivalent resistance lower.

The frequency is a function of your sampling rate, so slow rates have higher input resistacne and fast rates have lower input resistance.

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Please be positive and constructive with your questions and comments.