nice opamp

Not bad. $6.62 at Digi-Key, QTY 1:

--
Science teaches us to trust. - sw

The OPA858 is also quite nice. FET input is quite handy when I don't want to worry about input bias currents.

Yes, it's a very similar part. Less current noise, more voltage noise.

--

John Larkin      Highland Technology, Inc 

The best designs are necessarily accidental.

It only has less current noise below ~40MHz. Non constant over frequency current noise density of FETs was quite a surprise (disappointment?) when I first used the OPA858. There is no free lunch.

Did you measure it?

Datasheets:

858 Figure 11. Voltage Noise Density vs Frequency 855 Figure 11. Voltage and Current Noise Density vs Frequency

The 858 doesn't even spec current noise.

The 855 voltage noise levels out around 100KHz. The current noise settles around 2MHz.

Every device I know of has rising noise below some corner frequency that depends on each device. Why are you surprised?

--
Science teaches us to trust. - sw

Should have clarified I meant constant density above the 1/f corner freq.

Have a look at fig 53. in the OPA858 datasheet. The current noise density increases with increasing frequency.

That's standard with FETs--the drain circuit current noise couples back via C_DG.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

Perhaps driving the device from a low impedance will help cure the problem. After all, it is going to be hard to maintain a high impedance at 100MHz.

In fact, Figure 43. Noninverting Configuration and Figure 44. Inverting

from a low impedance around 50 Ohms.

In Figure 56. Transimpedance Amplifier Circuit on page 20 shows capacitances from various sources at the imput of the op amp. This will reduce the bandwidth to well below 100MHz.

The OPA855 is advertised as a transimpedance amplifier. They state:

When the device is configured as a transimpedance amplifier (TIA), the 8-GHz gain bandwidth product (GBWP) enables high closed-loop bandwidths at transimpedance gains of up to tens of kOhms.

So I guess I'd forget about the OPA858 and go with the OPA855.

--
The best designs occur in the theta state.

driven

Yes, I would not generally use the OPA858 if noise performance is the prior ity. It is still handy as a high bandwidth second stage amplifier after a l ow noise front end that has reasonable output impedance.

Yep I thought I had found a way to completely avoid shot noise from base bias currents of BJT's by using FETS (or pHEMTS ) but as you say the drain current noise is still a problem.