DSO Quad bandwidth

[geshsoft]

timbeurskens, LMH6626 is not even remotely close of a substitute for OPA2354 in this case. The specs are all wrong.

For instance, LMH6626 is not rail-to-rail rated op-amp which means that your dynamic range will suffer greatly.
The input bias current of LMH6626 is 7 orders of magnitude greater than OPA2354 which basically makes it unfit for this specific application (front end).
LMH6626 is only stable with gain > 10 and DSO quad uses configurations with gains lower than 10 which will more than likely lead to weird behavior (read oscillations and such)...
Simply putting a wide BWP amplifier without proper PCB layout can be quite interesting (ringing, noise, full out oscillations... you name it). I can only speculate what the PCB layout looks like on the DSO quad.

There isn't a simple fix for the limited BW of the DSO Quad. It has to be rather redesigned along with couple other things, such as implementing a real trigger...
I hope this was useful

[sagittarius0120]

Mark
!

[alienrelics]

I really wish I'd seen this thread before buying a DSO203.

Everyone is selling these as 72MHz analog bandwidth.

My analysis of the schematic (hardware 2.6 from the Wiki) is that the last Op Amps just before the ADC are single pole lowpass filters with a maximum -3dB frequency of 4.4MHz. That is about right for 36Msps per channel.

Using the built-in signal generator: On a 100MHz BW Tektronix TDS 220, the signal generator looks like a reasonably good square wave all the way up to 8MHz. With the Quad with only Channel A active, at 4MHz it looks like a sine wave with a minor flattening on the tips. At 6MHz, on the Tektronix it looks like a decent square wave, while on the Quad it is a sine wave with lower voltage. At 8MHz, on the Quad the amplitude is much lower.

So with my unscientific tests and past experience, I'd call this 4MHz analog bandwidth. I consider that useable for audio troubleshooting, and for digital signals no faster than 100kHz.

[alienrelics]

Any confirmation of bandwidth? I have one that is HW version 2.72. Does that mean it has two 72Msps channels, or is it an older one with two 36Msps channels?

[alienrelics]

A reviewer at EDN magazine says he measured a 3MHz bandwidth.

http://www.edn.com/design/test-and-meas ... under--200

[80631655974]

There is my upgrade!

[ingo2]

Hi,

My DS203 is harwdare version 2.73.
I just made the modification suggested by bielec:

For ch A.
Remove C73
C11=1.5pF
Across R17 3.3pF
Across R21 22pF
C9=1.5pF
Across R15 3.3pF
Across R19 12pF

For ch B.
Remove C74
C12=1.5pF
Across R18 2.2pF
Across R22 15pF
C10=1.5pF
Across R16 2.2pF
Across R20 12pF

I can confirm a bandwidth of about 11MHz at all input gain stages. This is the maximum possible bandwidth with the used amplifier. At low sensitivity higher bandwidth is possible.
At frequencies higher than 1 MHz there is notable jitter because of unstable trigger I think. Maybe this can be overcome with an improved software.

At the digital inputs I found that rectangle signals with an amplitude of 5V can be measured up to a frequency of about 4 MHz.

Greets

Ingo.

[alienrelics]

That's great. For a 72Msps scope, 11MHz is probably pushing it. Although I think later HW versions use two ADCs, can they double up on one channel and do 144Msps? That would make 11MHz bandwidth very usable.