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# THS4524: THS4524 Open Loop Gain

Part Number: THS4524
Other Parts Discussed in Thread: TINA-TI

Hello,

I was reading the datasheet on the THS4524 and have come across the "Open Loop Gain" parameter. I need to run some tests on the device and therefore am planning ahead of time on how to do so. I have never ran tests on a fully differential amplifier and the datasheet gives limited information on the Open Loop Gain. Are there any test conditions or a specific circuit example I could look at that might make it more clear.

Thank You

• Hi Youssef,

The open loop gain or "Aol" of the amplifier is the amplifier's gain with no feedback components connected. For an average op-amp, this is typically a very high value such as 120dB. Generally I would not suggest measuring the open loop gain directly unless you absolutely need to for some reason. When we measure it in production, we use compensation techniques that allow us to measure the open loop gain at DC. It is extremely challenging to measure it across frequency, and is not something we usually attempt to do outside of a simulation environment.

Regards,
• Hi Youssef,

the open loop gain of an OPAmp is not a "hidden" parameter which only plays a role in theory. A finite open loop gain has consequences which can be easily observed sometimes. Assume that an OPAmp produces a 1kHz sine signal of 1Vpp at its output and has an open loop gain of 60dB at this frequency. Then the OPAmp will only manage this with a 1kHz sine signal of 1mVpp between its input terminals. This 1mVpp sine signal can even be measured by the help of a scope, provided that not too much stray capacitance is introduced during the measurement.

The same is true for the THS4524. The following simulations give an idea of what is going on between the input terminals when the OPAmp shall produce a sine signal at its output. First the behaviour of a 10kHz sine is shown. The signal between the input terminals is first a zero line and can only be seen with a "zoom":

In this simulation the open loop gain at 10kHz appears to be about 81dB.

Now the behaviour at 100kHz:

The open loop gain at 100kHz appears to be about 61dB.

The following plot shows the "open loop gain" at other frequencies:

Strictly speaking, the plot shows the ratio of the signal amplitude between the input terminals and the signal amplitude of the input signal VG2, which is identical to the amplitude of the output signal here. To get the correct open loop gain, you still must reverse the sign.

Kai

• Hey Kai,

First I want to thank you for the detailed explanation.
I understand the theory behind it, but here you are assuming that the open loop gain is 60dB.

Also what is your voltage at VG2?

What I am trying to find out is how to measure the open loop gain in a DC method without using any signal generators or AC signals.

In the THS4524 datasheet right under "DC PERFORMANCE" , the first parameter is the "Open-Loop Voltage Gain (Aol)".
The minimum limit is 80dB. What I need to do is measure that Open-Loop Voltage Gain to verify that it truly is 80dB or higher (which i assume it will be higher since typical value is 116 dB).
Therefore my real question was how to measure that value and compare it to the Open-Loop Voltage Gain Value found in the datasheet.

Youssef

• Hi Youssef,

sorry, I forgot to attach the TINA-TI file:

youssef.TSC

VG2 is the green curve.

Kai

• Thanks to your explanation and circuit simulation I feel like I understand this device much better.