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 ...
DKNguyen's user avatar
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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 ...
Phil Frost's user avatar
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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 ...
bobflux's user avatar
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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 ...
LorenzoDonati4Ukraine-OnStrike's user avatar
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 ...
Spehro Pefhany's user avatar
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 ...
tobalt's user avatar
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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 ...
Andy aka's user avatar
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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=...
user57037's user avatar
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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 ...
Paul Elliott's user avatar
8 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, ...
Olin Lathrop's user avatar
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 ...
TonyM's user avatar
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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 ...
Andy aka's user avatar
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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 ...
Spehro Pefhany's user avatar
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 ...
bobflux's user avatar
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6 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 ...
比尔盖子's user avatar
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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 ...
Math Keeps Me Busy's user avatar
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 ...
pat's user avatar
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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 ...
Justme's user avatar
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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-...
Tim Williams's user avatar
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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. ...
jonk's user avatar
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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 ...
LorenzoDonati4Ukraine-OnStrike's user avatar
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 ...
Spehro Pefhany's user avatar
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 ...
Andy aka's user avatar
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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's user avatar
  • 7,603
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 ...
devnull's user avatar
  • 8,517
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 ...
Spehro Pefhany's user avatar
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 ...
DKNguyen's user avatar
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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 ...
Math Keeps Me Busy's user avatar

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