How about some suggestions for how to interpret these characteristics for a minimum input voltage?input offset voltage, bias current, input offset current
Not sure what you mean by "differential mode" but the following parameters all have some effect on an opamp circuit's performance with small signals, irrespective of how the amplifier is used:What parameters should I be looking at in a datasheet to determine the minimum input signal level an Op-Amp can handle in differential mode?
Noise figures I can see. The others, I don't see how they prevent a change in input signal from being recognized.Not sure what you mean by "differential mode" but the following parameters all have some effect on an opamp circuit's performance with small signals, irrespective of how the amplifier is used:
Input offset voltage and its behavior over temperature;
Input bias current and its behavior over temperature;
Input offset current and its behavior over temperature;
Input voltage noise density, together with system bandwidth
Voltage noise 1/f corner frequency;
Input current noise density, together with system bandwidth and input source resistance; and
Current noise 1/f corner frequency.
Those are the major factors to consider. Also, if there is any significant noise on the opamp's supplies, the power supply rejection ratio could be important.
The question is rather vague. That leaves a lot of options open for what could possibly interfere with his signal. For instance, if the input offset voltage is 5 mv and the input signal is 1 mv, the input signal might not cause a change of state because it does not overcome the input offset. The fact that you don't know this (and several other scenarios) is not a helpful answer.Noise figures I can see. The others, I don't see how they prevent a change in input signal from being recognized.
Re: Input offset voltageThe question is rather vague. That leaves a lot of options open for what could possibly interfere with his signal. For instance, if the input offset voltage is 5 mv and the input signal is 1 mv, the input signal might not cause a change of state because it does not overcome the input offset. The fact that you don't know this (and several other scenarios) is not a helpful answer.
I guarantee you, you'd see it real quick if you had an opamp with significant Vos, Ib and Ios temperature coefficients operating in an environment with fluctuating temperature. Even in a controlled room-temperature environment, like home or office, those tempcos can cause apparent input shifts that far exceed the opamp's voltage noise, often by more than an order of magnitude.Noise figures I can see. The others, I don't see how they prevent a change in input signal from being recognized.
Connecting the outputs of the bridge sensor directly to the opamp inputs won't work, because opamps have extremely high voltage gains that are way higher than what your application will need, plus they are highly unpredictable. They're meant to operate in a circuit in which feedback resistors are used to precisely set the circuit's voltage gain.I am interfacing an un-amplified bridge pressure sensor with the output from the sensor connecting directly to the inverting and non-inverting inputs of the Op-Amp.
I guarantee you, you'd see it real quick if you had an opamp with significant Vos, Ib and Ios temperature coefficients operating in an environment with fluctuating temperature. Even in a controlled room-temperature environment, like home or office, those tempcos can cause apparent input shifts that far exceed the opamp's voltage noise, often by more than an order of magnitude.
If a circuit has the amazingly good fortune to operate in an environment where the temperature never changes, or if the input offset voltage, input bias current and input offset current were somehow magically constant over temperature, you'd be correct; in that case, obviously, the only thing left to consider would be the opamp's noise specs.
No argument temperature changes and many things change with it. Does that mean an input signal would not be seen?
But in a real-world environment in which the temperature continually changes, and with real-world components whose input characteristics are temperature-dependent, those characteristics MUST be considered when designing systems that must process small signals.
It doesn't necessarily mean that an input signal would not be seen; but it does mean that unless the input signal is significantly larger than all the other stuff (noise, temperature-induced shifts, interference, etc.), you might not be able to say for certain that "Yes, THAT is a signal. It is definitely NOT noise or some other garbage."No argument temperature changes and many things change with it. Does that mean an input signal would not be seen?
Thanks for the Linear Tech. suggestions.Connecting the outputs of the bridge sensor directly to the opamp inputs won't work, because opamps have extremely high voltage gains that are way higher than what your application will need, plus they are highly unpredictable. They're meant to operate in a circuit in which feedback resistors are used to precisely set the circuit's voltage gain.
If you want a 1-chip solution that will allow you to connect the sensor directly and have a precise voltage gain, you need an instrumentation amplifier, such as an LT1167. It has excellent input specs and is designed for bridge amplifier applications.
Linear Tech also has an excellent application note on bridge measurement techniques that is well worth reading.
Could you clarify what you mean by this?there is no minimum signal that a differential op amp can handle, but you should review all op amp parameters..
Keep in mind that the amplifier you choose is not the only source of offset error: according to its data sheet, the MPX2300DT1 itself has a zero-pressure offset of +/- 750 μV, more than ten times the +/- 60 μV maximum offset of the LT1167.What I wasn't sure about is if the specifications state an input offset voltage in the mV range whether the Op-Amp would be suitable for use with a sensor that outputs a µV signal? For example, if I decided to use the LT1167 Instrumentation Amplifier, the input offset voltage is typically between 15-20µV however, the Freescale sensor mentioned about outputs a low µV signal.
LT1167? Good choice. Eve at maximum gain of 1,000 your offset error would be 0.06 V. Your input signal would ride on top of that. AC coupling, looking for a change in voltage, omits the offset error. Or the offset can be handled in software. Noise below 10 Hz is down around 0.28 uV. No problem there even at 5 V or 3.3 V power. Maximum output signal might be about 1.5 mV, plus offset.Connecting the outputs of the bridge sensor directly to the opamp inputs won't work, because opamps have extremely high voltage gains that are way higher than what your application will need, plus they are highly unpredictable. They're meant to operate in a circuit in which feedback resistors are used to precisely set the circuit's voltage gain.
If you want a 1-chip solution that will allow you to connect the sensor directly and have a precise voltage gain, you need an instrumentation amplifier, such as an LT1167. It has excellent input specs and is designed for bridge amplifier applications.
Linear Tech also has an excellent application note on bridge measurement techniques that is well worth reading.
AC coupling is almost never an option in pressure measurement, unless what's being measured is pressure variations.AC coupling, looking for a change in voltage, omits the offset error.
Read the data sheet. The LT1167 is specified to operate only down to +/- 2.3 volts supply, or 4.6 volts total, and will not work at 3.3 volts. If the LT1167 could operate from a 3.3 volt supply, the manufacturer would have said so.No problem there even at 5 V or 3.3 V power.
Super!. No problems at those voltages.AC coupling is almost never an option in pressure measurement, unless what's being measured is pressure variations.
Read the data sheet. The LT1167 is specified to operate only down to +/- 2.3 volts supply, or 4.6 volts total, and will not work at 3.3 volts. If the LT1167 could operate from a 3.3 volt supply, the manufacturer would have said so.
I don't understand what you mean by that. Explain, please?Super!. No problems at those voltages.
I see no problems with your design working at +/- 2.3 or so volts.I don't understand what you mean by that. Explain, please?
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