Gerd, I completely agree with you. Nevertheless, the following excerpt from the data sheet may help understanding the instability problems:
DRIVING CAPACITIVE LOADS One of the most demanding and yet very common load conditions for an op amp is capacitive loading. Often, the capacitive load is the input of an analog-to-digital (A/D) converterçincluding additional external capacitance that may be recommended to improve the A/D converter linearity. A high-speed, high open-loop gain amplifier like the OPA2674 can be very susceptible to decreased stability and closed-loop response peaking when a capacitive load is placed directly on the output pin. When the amplifier open-loop output resistance is considered, this capacitive load introduces an additional pole in the signal path that can decrease the phase margin. Several external solutions to this problem have been suggested. When the primary considerations are frequency response flatness, pulse response fidelity, and/or distortion, the simplest and most effective solution is to isolate the capacitive load from the feedback loop by inserting a series isolation resistor between the amplifier output and the capacitive load. This does not eliminate the pole from the loop response, but rather shifts it and adds a zero at a higher frequency. The additional zero acts to cancel the phase lag from the capacitive load pole, thus increasing the phase margin and improving stability. The Typical Characteristics show the Recommended RS vs Capacitive Load and the resulting frequency response at the load. Parasitic capacitive loads greater than 2pF can begin to degrade the performance of the OPA2674. Long PC board traces, unmatched cables, and connections to multiple devices can easily cause this value to be exceeded. Always consider this effect carefully, and add the recommended series resistor as close as possible to the OPA2674 output pin (see the Board Layout Guidelines section). BOARD LAYOUT GUIDELINES Achieving optimum performance with a high-frequency amplifier like the OPA2674 requires careful attention to board layout parasitic and external component types. Recommendations that optimize performance include: a) Minimize parasitic capacitance to any AC ground for all of the signal I/O pins. Parasitic capacitance on the output and inverting input pins can cause instability; on the noninverting input, it can react with the source impedance to cause unintentional band limiting. To reduce unwanted capacitance, a window around the signal I/O pins should be opened in all of the ground and power planes around those pins. Otherwise, ground and power planes should be unbroken elsewhere on the board. 73, Ahti OH2RZ On 10/12/2008, Gerd Loch <[EMAIL PROTECTED]> wrote: > I have accidentially made all sorts of mismatching the output from my > OPA2674 and never have blown it. > Running audio with Ozy/Janus and more than 1W rf-output. Maybe the reason is > that you have overdriven the audio input. > > Gerd, DJ8AY > > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Pete Ferrand > Sent: Wednesday, December 10, 2008 4:16 AM > To: Ahti Aintila > Cc: flexradio@flex-radio.biz > Subject: Re: [Flexradio] blown SDR-1000 final opamp > > > > Ahti: > > Thank you for taking the time to reply. My radio has the OPA2674 and that is > what it had originally. The issue is not one of a short circuit, but most > probably one of dealing with a high impedance load which would send the > voltage up beyond what the part can handle. At least that's what I'm > guessing; I'm not sure of the correct troubleshooting procedure for this > kind of fault. > > My hope was that someone has figured out how to make the circuit more robust > but since yours was the only reply and there's no mention of it in the > archives I suppose that hasn't happened. > > 73, > -Pete > WB2QLL > Somers, WI > > > > > > > > > > -----Original Message----- >>From: Ahti Aintila <[EMAIL PROTECTED]> >>Sent: Dec 9, 2008 4:54 AM >>To: Pete Ferrand <[EMAIL PROTECTED]> >>Cc: flexradio@flex-radio.biz >>Subject: Re: [Flexradio] blown SDR-1000 final opamp >> >>Pete, >>The very old SDR-1000 units have OPA2677 as the final amplifier and >>cannot stand short circuit. The newer radios use OPA2674 that has >>internal short circuit protection. Anyhow, ask Flex people directly, >>they know better the difficulties with some of the first production >>units. >> >>73, Ahti OH2RZ >> >>On 09/12/2008, Pete Ferrand <[EMAIL PROTECTED]> wrote: >>> >>> Some months ago I blew the final opamp in my 1W SDR-1000 whilst >>> matching it to a linear. Wasn't happy about that but since I >>> accidentally hooked up the attenuator resistors wrong I wasn't too >>> surprised since the rig saw a very high impedance. Worked fine ever >>> since. >>> >>> Unfortunately I'm less happy about blowing the amp this morning, >>> about a half year after that first incident, whilst trying it on six >>> meters for the first time. Without the linear which doesn't cover >>> six. Just wanted to see if I could work a couple locals. As far as >>> the MFJ-269 showed there was a perfect match into the ATU but when I >>> increased the power beyond about a tenth of a watt output the opamp >>> popped again. >>> >>> Besides the time and money aspect, the circuit board clearly can't >>> take a lot of parts replacements. >>> >>> Has anyone figured out how better to protect this part? Or some >>> better solution. This is a four stack including the rfe board, older >>> version with nylon spacers. >>> >>> Thanks. >>> >>> -Pete >>> WB2QLL >>> Somers, WI >>> >>> >>> >>> >>> _______________________________________________ >>> FlexRadio Systems Mailing List >>> FlexRadio@flex-radio.biz >>> http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz >>> Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ >>> Knowledge Base: http://kc.flex-radio.com/ Homepage: >>> http://www.flex-radio.com/ >>> > > > _______________________________________________ > FlexRadio Systems Mailing List > FlexRadio@flex-radio.biz > http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz > Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ > Knowledge Base: http://kc.flex-radio.com/ Homepage: > http://www.flex-radio.com/ > > _______________________________________________ FlexRadio Systems Mailing List FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ Knowledge Base: http://kc.flex-radio.com/ Homepage: http://www.flex-radio.com/
