Question about ADC

hau30729

Question about ADC [Copy link]

 

The software uses ADC (ESP-12) to read the voltage and it reads 1V, but when it is measured with an ammeter or oscilloscope, it is only 0.5V. At this time, the voltage read by ADC will also become 0.5V (but the voltage is not adjusted in the process). What may be the reason for this situation?

littleshrimp

The voltage source has high output impedance and the oscilloscope has low input impedance, which causes the voltage to be pulled down. It is recommended to add buffering

littleshrimp

The voltage source has high output impedance and the oscilloscope has low input impedance, which causes the voltage to be pulled down. It is recommended to add buffering

littleshrimp

The voltage source has high output impedance and the oscilloscope has low input impedance, which causes the voltage to be pulled down. It is recommended to add buffering

littleshrimp

The voltage source has high output impedance and the oscilloscope has low input impedance, which causes the voltage to be pulled down. It is recommended to add buffering

hau30729

littleshrimp posted on 2019-10-31 11:48 The voltage source has a high output impedance and the oscilloscope has a low input impedance, which causes the voltage to be pulled down. It is recommended to add buffer processing

Does it mean that a Burffer like 7407 needs to be added between the ADC of the MCU and the voltage being read?

maychang

hau30729 posted on 2019-10-31 13:12 Does it mean that a burffer like 7407 needs to be added between the ADC of the MCU and the voltage being read??

The voltage to be measured is an analog quantity. How can a digital chip like 7407 be used? An operational amplifier should be used.

guitianxia

No need to add

hau30729

guitianxia posted on 2019-10-31 23:04 No need to add

What does it mean that you don't need to add it?

hau30729

maychang posted on 2019-10-31 16:10 The voltage to be measured is an analog quantity. How can it be done with a digital chip like 7407? An operational amplifier should be used.

Understood, I just thought that 7407 is a commonly used buffer, and I would use OPA to connect it by analogy.

maychang

hau30729 posted on 2019-11-1 14:27 I understand. I just thought that 7407 is a commonly used buffer. By analogy, I will use OPA to connect it.

The voltage drops when connected to an oscilloscope or electric meter, which can only be because the internal resistance of the signal source is very large, as mentioned on the second floor.

When adding buffers, you need to consider the input resistance of the op amp, and you cannot use an op amp with a lower input resistance. For example, the NE5532 has a typical input resistance of 300kΩ and a minimum of only 30kΩ, which cannot be used in your circuit.

xiaxingxing

maychang posted on 2019-11-1 17:31 The voltage drops when connected to an oscilloscope or meter. It can only be that the internal resistance of the signal source is very large, as mentioned in the second post. Adding buffers requires considering the input voltage of the op amp...

"At this time, the voltage read by ADC will also become 0.5V" Sir, why does the voltage read by ADC later also become 0.5V?

At first, the voltage read by ADC was 1V. Thank you!

maychang

xiaxingxing posted on 2020-4-1 22:27 "At this time, the voltage read by ADC will also become 0.5V" Senior, why does the voltage read by ADC later also become 0.5...

I guess the original poster first used the ADC (ESP-12) to read a 1V voltage, and then connected the meter or oscilloscope in parallel to the ADC (ESP-12) input terminal. At this time, the meter or oscilloscope input resistor is connected to the signal source to be measured, and the voltage at the signal source end is pulled down. Obviously, at this time, the meter or oscilloscope reading and the ADC (ESP-12) reading are both the signal source end voltage after being pulled down.