21
votes
Accepted
How OPA333 withstand 400V bus without burning in this current sensing application?
The OPA333 isn't referenced to ground. It is referenced to the 400V supply bus via the zener diode and Rz. Well, technically the OPA333 is using 400V-5.1V as it's reference, but that reference is ...
- 57k
19
votes
How does an op-amp know where ground is?
How does anything know where ground is? Ground is just a symbol we stick on the schematic to make it easier to read. None of the components in a normal circuit read the schematic, so none of them know ...
- 57.8k
17
votes
Accepted
What exactly happens to the signals hitting a common mode choke?
Adding to Andy's answer, no need to repeat what he wrote.
From what you write, I think your problem is more about intuitively understanding how the choke works. Consider an inductor:
This inductor ...
- 80.3k
15
votes
What exactly happens to the signals hitting a common mode choke?
For ordinary common mode chokes, the differential mode impedance reduces essentially to the wire resistances, whereas the common mode impedance is largely inductive, with the wire resistance as a ...
11
votes
Can an op-amp's Vin+ and Vin- be outside the supply voltage as long as their difference is inside it?
Generally speaking, no. The datasheet should clearly tell you what is permissible without damage and what might work and what is guaranteed to work (six different numbers in general).
Both inputs of ...
11
votes
I designed and built an op-amp using 4 NPN and 2 PNP BJTs. What are the pros and cons, and how can it be improved?
This is basically only the input part of what usually one considers to be an operational-amplifier, also called operational transconductance amplifier (OTA).
One obvious downside is, that the output ...
- 22.8k
10
votes
What exactly happens to the signals hitting a common mode choke?
For normal differential currents, the two windings effectively "subtract" their inductances and thus current is barely impeded.
When the current is common mode, the full inductance of both coils is ...
- 467k
9
votes
What is a common mode signal in a layman's language and why do we care for it while studying Differential Amplifiers?
Example to motivate the answer
Imagine a microphone at the end to two long wires. You connect these two wires to the microphone input of a amplifier. What comes out of the speakers is a lot of hum, ...
- 314k
9
votes
How OPA333 withstand 400V bus without burning in this current sensing application?
No part of the op-amp is grounded. The whole op-amp is referenced to the 400V rail. The positive supply terminal of the op-amp is 400V. Because of the 5.1V Zener diode, the negative supply is 400-5.1=...
- 29.4k
8
votes
Understanding common "mode"
"Common-mode" has absolutely nothing to do with mean, median, mode, or statistics in general. Instead, it means that a common signal is present in a differential circuit. And "differential" has ...
- 1,000
8
votes
Can an op-amp's Vin+ and Vin- be outside the supply voltage as long as their difference is inside it?
No, op-amps cannot operate correctly with either of their inputs outside of their supply rails.
The internal transistors must be operating from a supply higher than their base/gate voltage to function ...
- 23.7k
8
votes
Accepted
Common-mode choke with inputs shorted together
It'll act like an inductor with an inductance equivalent to one of the windings. Imagine both inductors as bifilar windings like this: -
Image from here. If you think about it, it might as well be ...
- 467k
7
votes
What is "common-mode" noise?
Motivations
"Common-mode noise" is a fundamental concept in electronics. Yet, because it appears in a lot of related but different contexts - amplifiers and signaling, 120 V or 230 V power ...
- 7,267
7
votes
How does an op-amp know where ground is?
The input bias currents behave as below, where I1 and I2 are the respective input bias currents and I2-I1 is the input offset current.
simulate this circuit – Schematic created using ...
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 ...
- 80.3k
6
votes
Why does this op amp input stage have single ended output?
If you look carefully, you will notice that the collector loads of the differential pair (quad?) are a current mirror (Q8, Q9). Whenever you have that combination of a differential pair with a current ...
- 26.1k
6
votes
How is the input offset voltage a function of common mode voltage?
As far as I understand, if the Vcm varies the Voff varies and this would cause non linearity error at the output Vout.(Is that true?) Yes.
And is it possible to explain how the common mode voltage ...
- 2,235
6
votes
Accepted
Ground loop and oscilloscope isolated from earth
There are several mechanisms at play, and it does not even depend on specific devices like an oscilloscope, it applies to all devices, such as desktop computers and even humans.
First of all, even if ...
- 164k
6
votes
Current Rating in Common Mode Choke
A search on the manufacturer's website would prove fruitful:
EMC Products based on Nanocrystalline VITROPERM | VACUUMSCHMELZE
we find Saturation Behavior on page 10.
CMCs are also known as current-...
- 43.6k
5
votes
How does an op-amp know where ground is?
First of all, ground is the arbitrarily chosen point where you reference all the voltages in the circuit. In the most common simple circuit configurations ground is chosen either as the negative ...
5
votes
Accepted
NPN Differential Pair - Emitter Voltage Variation
Mario mentioned a thought about this. But perhaps this doesn't actually answer your question about why, in your single-ended case, you get half the AC peak-to-peak voltage at the shared emitter node.
...
- 78.5k
5
votes
Accepted
High-side current sensing with a rail-bound instrumentation amplifier?
A better way is to use a rail-to-rail input precision op-amp and a transistor.
simulate this circuit – Schematic created using CircuitLab
Do an error budget calculation on your proposed ...
5
votes
Accepted
Which diagram is better for common mode noise rejection?
The second diagram is better because the differential inputs don't share a wire with the power supply feed providing 0 volts to the sensor. That 0 volt feed will take current and, in the simple DC ...
- 467k
5
votes
Accepted
Opamp input offset voltage...common mode?
The circled expression is under the "Test Conditions" heading so it is describing the conditions under which the input offset voltage was measured.
V+ is the name of the positive power supply pin:
...
Null♦
- 7,633
5
votes
OpAmp with common-mode voltages and zero feedback current
Actually with finite gain [...] the argument would be even more compelling - finite open loop gain multiplied by zero potential must be zero!
I find it even more clear to see with finite gain. You ...
- 8,567
5
votes
Do differential ADC's really reject 100% of common mode noise?
A theoretical perfect differential ADC (or differential amplifier) will reject 100% of common-mode noise regardless of magnitude or bandwidth.
Now compare the CMRR specification on the datasheet of a ...
5
votes
Can an op-amp's Vin+ and Vin- be outside the supply voltage as long as their difference is inside it?
Definition: A channel is the hardware input on the scope a probe plugs into.
I understand professional oscilloscopes can accommodate this, what circuitry exists in oscilloscopes that allows ...
- 57k
5
votes
Can an op-amp's Vin+ and Vin- be outside the supply voltage as long as their difference is inside it?
Typically, op-amps do not work properly if either of the inputs lies outside of the range of the power rails. [In fact, many op-amps are not rail-to-rail, and their proper operating range is smaller ...
- 26.1k
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