21
votes
Accepted
Instrumentation Amplifier with a gain of 1 doesn't output a different signal
While the datasheet of the INA828 seems ambiguous regarding acceptable input voltage range, it is clear that you can't use inputs so close to the negative rail. The first hint, on page 19 is:
7.3.3 ...
- 47.3k
17
votes
mV variations even with a simple voltage divider
There are plenty of possible sources for noise, and all of them are pretty much to be expected. Real circuits have to deal with reality, and reality has a lot of things that simulators often ignore.
...
- 73.6k
15
votes
Why aren't these instrumentation amps working for linearly driving a generic MOSFET?
None of these chips are designed to operate on a +/-24V supply.
Some of the circuits have 0V between VDD and VSS, so will not work.
All of the circuits use a topology that requires high gain to ...
14
votes
Why are two stages used for an instrumentation amplifier?
One of the biggest benefits of the 3 op amp INA is the equal and high input impedance. The op amp's non-inverting pins' input impedance can be up in the \$T\Omega\$ range. I'll leave it as an exercise ...
- 13.9k
14
votes
Accepted
Why are two stages used for an instrumentation amplifier?
The 3 op-amp design has three main advantages over a single op-amp differential amplifier.
The input impedance is much higher, since the inputs drive directly into an op-amp input rather than into a ...
- 23.1k
13
votes
Accepted
How to avoid Johnson noise in high input impedance amplifier
The problem in your reasoning is that you do not show the complete path of the signal. More specific the impedance level of the signal.
You are right in that you cannot have both a high impedance and ...
- 81.4k
13
votes
Accepted
Why include a small resistance between op-amp stages
The OPA633 has a peak in its frequency response at about 200 MHz of nearly 5 dB when the signal source impedance is 50 ohm. If the source impedance is about 300 ohm that peak is about 1 dB. If you ...
- 473k
13
votes
Active guard shielding for instrumentation amplifier
[This is a more theoretical addendum to the answer by @VoltageSpike. ]
Active driven guard is used when the signal source (the sensor) has a high output impedance. The amplifier input has a (very) ...
- 38.6k
13
votes
Accepted
Do you have to use an instrumentation amplifier to measure voltage across a 0.01 ohm shunt?
You don't have to use a differential amplifier providing you are careful about the voltage drops that occur along the grounds. If you return the bottom end of the 500 ohm resistor to the same point as ...
- 34.2k
13
votes
Do you have to use an instrumentation amplifier to measure voltage across a 0.01 ohm shunt?
You have an amplifier with a gain of about +2000 and an offset voltage of as much as +/-3mV at room temperature. It might typically be +/-2mV (onsemi datasheet). Since the output swing with no load ...
12
votes
Instrumentation Amplifier with a gain of 1 doesn't output a different signal
You are operating outside of the common mode input range. From page 5 of the datasheet:
- 2,912
10
votes
Circuit of Instrumentation Amplifier
In short, with the split and grounded R1 circuit, A1 and A2 amplify both common and differential mode signals by the same gain.
simulate this circuit – Schematic created using CircuitLab
So ...
- 3,242
10
votes
Accepted
How to implement gain and offset in a single opamp?
Let's look at a simplified diagram:
simulate this circuit – Schematic created using CircuitLab
Let's set \$R_f=68\:\text{k}\Omega\$.
You know that \$1+\frac{R_f}{R_{th}}=3.5\$, which is your ...
- 78.7k
9
votes
RC Lowpass Filter between Amplifier and ADC input
If you are reading the ADC just once per second then you need to eliminate frequencies above 0.5 Hz to prevent aliasing. If you think your system will have noise at, say 10 Hz, then that noise will ...
- 31.7k
9
votes
Circuit for small level voltage detection (microvolts)
Although it is possible to design a Sallen Key filter with a gain higher than unity, this is rather uncommon for a reason. Any gain in it introduces positive feedback into the structure and leads it ...
- 8,546
9
votes
Accepted
What is an instrumentation amplifier and how does the AD524 work?
An instrumentation amplifier is a difference amplifier (your orange box) with buffered inputs (your blue and green boxes) to achieve higher input impedance than a simple op amp difference amplifier1. ...
Null♦
- 7,652
8
votes
Accepted
How do I amplify a 0-100mV signal to an ADC with a range from 0 to a specific reference voltage?
When you really need to get to ground on the output so that even a "rail to rail" output isn't good enough, give it negative power. A opamp to drive a A/D input doesn't need much current, so a charge ...
- 315k
8
votes
Accepted
Why do tube amplifiers have such (relatively) small reservoir capacitors?
The energy stored in a cap is proportional to the square of the voltage -- so a 47uF cap on a 500V rail is storing as much energy as a 4700uF cap on a 50V rail. That's going to be part of it.
...
- 47.1k
8
votes
Circuit for small level voltage detection (microvolts)
The opamp in a Sallen-Key filter is supposed to be a unity-gain buffer. Yours has a gain of +3, so it isn't surprising that it's oscillating. Wikipedia talks about this.
If you need that much gain, ...
- 178k
8
votes
Accepted
K-type thermocouple, instrumentation op-amp and Arduino
You need a DC path for the in-amp bias currents, for example you could ground the junction or connect one lead or both to ground through a relatively high value resistor (the thermocouple and leads ...
8
votes
Active guard shielding for instrumentation amplifier
If the objective is to shield from RF, then its only necessary to connect the shield on one end to the ground of the PCB or chassis ground (and not on the sensor end which would make a ground loop). ...
- 88.8k
8
votes
Accepted
How to make this arbitrary waveform?
The reflection about the Y axis can be obtained by inverting one of the input signals but not the other. Look carefully at the summing junctions in the following circuit, noting how the first (top) ...
- 47.3k
8
votes
Why would I want to use a differential op amp configuration while amplifying differential signals?
Your question is "philosophical" and therefore requires a "philosophical" answer:-) You are actually asking, "What is the idea behind the classic op-amp instrumentation ...
- 19.5k
7
votes
How do I amplify a 0-100mV signal to an ADC with a range from 0 to a specific reference voltage?
If you look at section 28.6.3 of the ATmega328P data sheet you will see that it begins to define the zero offset error that you will get when using the ADC. The upshot of this is that it is extremely ...
- 473k
7
votes
Accepted
What is the best way to generate a negative supply rail using a microcontroller?
A common method for generating a negative supply rail for operation- and instrumentation- amplifiers (which don't require a lot of current) is with a capacitive charge pump. A classic part for this ...
- 38.6k
7
votes
Accepted
How to amplify a tiny signal riding on a large common mode square wave
Common mode changes slowly, signal changes fast.
What you thus need is a high-pass filter, which filters out the DC component. In the easiest case: that's a capacitor in series with your signal ...
- 100k
7
votes
Accepted
Instrumentation amplifier noise amplified
LM358 has very low gain-bandwidth product (1MHz) therefore the Sallen-Key filter topology you used might not work well as a lowpass above a few tens of kHz. What happens is the HF jumps over the opamp ...
- 81.9k
7
votes
Accepted
How to determine op-amp gain with active feedback?
What you need to do is add a couple of resistors
simulate this circuit – Schematic created using CircuitLab
If the instrumentation amp has a gain G, then, since the current through R1 must ...
- 61k
7
votes
How to make repeatable Op-amp amplifier circuit
Look at section 8.10 in the data sheet. The part you have chosen to use has an input offset voltage of \$\pm 0.3\mathrm{mV}\$ typical, \$\pm 2\mathrm{mV}\$ worst case. Your output offset voltage in ...
- 47.1k
7
votes
K-type thermocouple, instrumentation op-amp and Arduino
You misunderstand how a thermocouple works. A thermocouple generates a voltage that is more or less proportional to the difference in temperature between the junctions. If your "hot" junction is at ...
- 47.1k
Only top scored, non community-wiki answers of a minimum length are eligible