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https://www.ti.com/lit/gpn/OPA855
jla...@highlandsniptechnology.com wrote:
https://www.ti.com/lit/gpn/OPA855
Not bad. $6.62 at Digi-Key, QTY 1:
https://octopart.com/search?q=OPA855¤cy=USD&specs=0
--
Science teaches us to trust. - sw
On Thursday, December 10, 2020 at 5:19:12 PM UTC+11, Steve Wilson wrote:
jla...@highlandsniptechnology.com wrote:
https://www.ti.com/lit/gpn/OPA855
Not bad. $6.62 at Digi-Key, QTY 1:
https://octopart.com/search?q=OPA855¤cy=USD&specs=0
--
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.
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.
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.
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.
Yes, I would not generally use the OPA858 if noise performance is the priority. It is still handy as a high bandwidth second stage amplifier after a low noise front end that has reasonable output impedance.Dominic Chan <domini...@gmail.com> wrote:
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.
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
Configuration (Gain = â7 V/V) on page 15, both show the device driven
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.
On 12/10/20 10:01 PM, Dominic Chan wrote:
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