THS7530: Clipping on high input amnplitudes?!

Hendrik Funke

Part Number: THS7530

Hello,

we built up an ultrasound RX path with two THS7530 in series and are now faced with the following (ugly) behaviour. The configuration is single ended to differential, this is my input signal at pin Vin+:

With minimum gain (Vg+- = 0V -> 11.6 dB) I'm expecting a sine with around 4 Vpp on the outputs, but at pin Vout- I only get this:

The THS7530 clips my sine and I don't now why. I can't find any limitations regarding the input voltage in the datasheet, except in section "DC performance". The output clamp circuit isn't the problem, these voltages are set to 1.5V and 3.5V. By increasing the gain I get this:

Is this a known behaviour and can you reproduce this with your eval boards?

Thanks and Best Regards

Hendrik

over 6 years ago

0 kai klaas69 over 6 years ago

Guru 140200 points

Hi Hendrik,

can you show us a schematic?

Kai

0 Hendrik Funke over 6 years ago in reply to kai klaas69

Prodigy 70 points

Hi Kay,

shure, I forgot. This is from my test board.

Load is 200 Ohms differential. I get the same behaviour with differential input.

Thanks

0 kai klaas69 over 6 years ago in reply to Hendrik Funke

Guru 140200 points

Hi Hendrik,

you should considerably decrease your input signal. And what is at the VG+ pin? Shouldn't this be a voltage divider?

Kai

0 Sean Cashin over 6 years ago in reply to Hendrik Funke

TI__Genius 17756 points

Hi Hendrik,

The output looks as if the voltage clamp of the first stage is clipping the signal for some reason. However, it is difficult for me to analyze this circuit without seeing both stages and seeing the voltages and resistor values.

If you are unable to share more of your schematic on E2E, please email me directly at s-cashin@ti.com.

Best regards,

Sean

0 Hendrik Funke over 6 years ago in reply to Sean Cashin

Prodigy 70 points

Hi Sean,

thanks for your help. The curves in my first post are directly taken at the output pins of the first stage, so the output clamping shouldn't be the problem. The second stage behaves the same, when driven with a high input signal from the first stage. With 400 mVpp input voltage everything is fine, above the clipping starts (I measured the distortion with spectrum analyzer).

The schematic shows the first stage, I just deleted the output-circuitry.

It should be easy to reconstruct the setup with an THS7530 eval board.

Hendrik

0 kai klaas69 over 6 years ago in reply to Hendrik Funke

Guru 140200 points

Hi Hendrik,

what you observe is quite usual for a VGA: The input signal must not be too high.

Kai

Because of the Thanksgiving holiday in the U.S., TI E2E™ design support forum responses may be delayed from November 25 through December 2. Thank you for your patience.

Amplifiers

Amplifiers forum

THS7530: Clipping on high input amnplitudes?!