I am trying to convert a signal from accelerometer to a digital value. The output coming from accelerometer is in mv so I have added a non inverting voltage amplifier, then too I am getting a dc voltage at the output of opamp so I have added two capacitors to eliminate dc voltage in my signal. Here are my issues,

When I tried have connected a potentiometer to pin A1 and tried, I didn't find any issues, ADC is working perfectly but when I am trying to connect the output from the capacitor to PA1 and ground, I could not able to get the reading from controller!!!! I mean the voltage across capacitor is dropping to mV . I have measured the voltage across capacitor in oscilliscope by not connecting to PA1 its working perfectly, peak to peak is coming at 4V when vibrating.. but the moment I connect the ouput from capacitor to PA1 and measure the voltage across PA1 to GND. its dropping to mV, because of which i cannot do the data acquisition.

the source driving is output from signal conditioner which provide a constant power to industrial accelerometer. http://www.pcb.com/products.aspx?m=482c05


Are there any hardware connections/ biasing I need to do for conversion of an AC signal?

I am using a STM32F429 discovery board, Keil, ADC is in interrupt mode. Kit is powered through USB via computer.
Below is the link for schematic
Sensor is below link
http://www.pcb.com/Products.aspx?m=601A02
MOD: transferred your image.

https://drive.google.com/open?id=10FeMGhZmuvYHY8jgEu6LIBO_OmTPIElD

 

Hello,

The problem is with the single power supply.
When you put the ground of R3 to 1/2 the powersupply voltage, you will have a virtual gound.

Bertus

 

The capacitors are b;locking the DC output . connect pin 1 op amp to the ADC

 

The capacitors are b;locking the DC output . connect pin 1 op amp to the ADC

I do not want any DC voltage driving to my input of microcontroller, so I have place a capacitor, If i connect directly to pin 1 it will give false values,

 

Hello,

The problem is with the single power supply.
When you put the ground of R3 to 1/2 the powersupply voltage, you will have a virtual gound.

Bertus

Any solution please?

 

hi ashok,
Post details of the mV source, what is driving it.?
E

 

hi ashok,
Post details of the mV source, what is driving it.?
E

http://www.pcb.com/Products.aspx?m=601A02
The above link, accelerometer output.

 

hi,
Is this specification data for the Acc type you are using.?
Note the excitation and the Vout Bias voltage.!!!
E

 

Hello,

How is the sensor itself connected?
Is there a blocking capacitor at the output of the sensor as given in the datasheet?

As for the amplifier, try this circuit:



https://www.radio-electronics.com/info/circuits/opamp_non_inverting/op_amp_non-inverting.php

Bertus

 

You need to download the Product Manual for this accelerometer and study the General Operating Guide section CAREFULLY so you can learn how to use it properly.

In any case, you absolutely CANNOT capacitively couple a signal-- ANY signal-- into an ADC input without also providing a DC path for the ADC's input bias current-- either a resistor to circuit common, or a voltage divider between the ADC's Vref and circuit common so as to bias the ADC inputs to some DC level. The circuit diagram you posted has no such DC path, therefore the ADC will not work properly.

 

Hello,

The circuit I posted will bias at 1/2 VCC when the resistors R3 and R4 are equal.
Use no capacitor at the output, as @OBW0549 said.

Bertus

 

Please show the complete circuit with the accelerometer

 

hi,
Is this specification data for the Acc type you are using.?
Note the excitation and the Vout Bias voltage.!!!
E

I have connected a signal conditioner which takes care of the excitation voltage of sensor Eric, yes that is the specification sheet i am working on.

The sensor gets the excitation voltage from signal conditioner, and to eliminate dc bias voltage i have connected the capacitor in series at the output of op amp
http://www.pcb.com/products.aspx?m=482c05

 

I think others have covered most of the problems with the circuit, but I'll add one point:

When you AC couple into the ADC input you will have signals going negative with respect to "ground" for the ADC. If the ADC has protection diodes, this may be permissible, though the current transients can be quite high if the output of the op amp falls rapidly. Even if the protection diodes keep the input from being damaged, any such negative voltage may cause malfunction of other channels if the ADC is multiplexed.

 

Even if the protection diodes keep the input from being damaged, any such negative voltage may cause malfunction of other channels if the ADC is multiplexed.

That's a very good point, and I have personal experience that says it's pertinent: while using a PIC18F1320 I found that while making conversions on one ADC input, I'd get large errors if any of the protection diodes on other inputs were forward biased.

It's a good idea to ALWAYS keep analog (and digital) inputs between Vss and Vdd. Otherwise, strange things have been known to happen with one of these chips.

 

Performance ENGLISH SI Channels 4 4 Sensor Input Type(s) ICP® ICP®
Output Range(Maximum) ± 10 V ± 10 V

How are you handling the output voltage swing ? ± 10 V

 

hi ashok,

If you use a rail2rail OPA eg:MCP6001/02 and a single supply voltage supply of +5V, this circuit will connect to the ADC input.
Vso swing about +2.5v so, no negative under swings and as the supply is only 5V , no over swing.
E

 

± 10 V

Hi Walter, that is the maximum output from a signal conditioner. I have never experienced an output of 10V from the signal conditioner. I have seen 5V only when I have applied a huge amount of sudden force !

 

ICP accelerometers are "phantom powered" using a constant current source which presents a high impedance to the signal. This results in an DC bias on the signal that is reasonably constant but of uncertain magnitude. (I've designed and built circuitry for such transducers.)

The DC offset can be blocked with a capacitor if the time constant of the resulting circuit is adequate to allow the lowest frequency of interest with tolerable error. If capacitive blocking is unacceptable, then there is little alternative but to produce an adjustable DC offset nulling circuit. Regardless of the method, by the time the signal gets to the ADC input it must have a DC offset that is approximately at the midpoint of the ADC's input range (e.g. 2.5 V for a 0 to 5 V ADC input). Since acceleration is the differential of velocity with respect to time, reduction in velocity will produce a negative-going signal. Presumably this information is not unimportant.

 

hi ashok,

If you use a rail2rail OPA eg:MCP6001/02 and a single supply voltage supply of +5V, this circuit will connect to the ADC input.
Vso swing about +2.5v so, no negative under swings and as the supply is only 5V , no over swing.
E

Can I use LM324 in place of MCP6001/02?