Tech Note on Op-Amps Driving Capacitance

A great tech note on OP Amps driving capacitance.

Q. How does capacitive loading affect op amp performance?

A. To put it simply, it can turn your amplifier into an oscillator. Here’s how:

Op amps have an inherent output resistance, Ro, which, in conjunction with a capacitive load, forms an additional pole in the amplifier’s transfer function. As the Bode plot shows, at each pole the amplitude slope becomes more negative by 20 dB/ decade. Notice how each pole adds as much as -90° of phase shift. We can view instability from either of two perspectives. Looking at amplitude response on the log plot,circuit instability occurs when the sum of open-loop gain and feedback attenuation is greater than unity. Similarly, looking at phase response, an op amp will tend to oscillate at a frequency where loop phase shift exceeds -180°, if this frequency is below the closed-loop bandwidth. The closed-loop bandwidth of a voltage-feedback op amp circuit is equal to the op amp’s bandwidth product (GBP, or unity-gain frequency), divided by the circuit’s closed loop gain (ACL).

More at the original article at


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Join me at VCF East Friday April 17th

I will be doing a workshop this Friday at VCF East on computer architecture. Each year is bigger and better and this is the second year for three days of computer related festivities. Worth the drive if you are in the NJ, NY, PA area!




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Thermocouple Amplifiers

I just did a video for Hackaday about Thermocouple Amplifiers, a quickie video feature Linear Technology in this case.  The spares from the build for the video are available at along with a Type K Thermocouple.

From the Hackaday video

Thermocouple is a terrific way to measure temperature. The effects of temperature change on dissimilar metals produces a measurable voltage. But to make that measurement you need an amplifier circuit designed for the thermocouple being used.

Linear Technology LTC 1049 Low Power Zero-Drift Operational Amplifier with Internal Capacitors

Linear Technology LTC 1049 Low Power Zero-Drift Operational Amplifier
with Internal Capacitors

While researching “Zero Drift Amplifiers” as a follow-up to my video on Instrumentation Amplifiers I noticed the little schematic the front page of theLTC1049 datasheet which is shown here. I thought it was an ideal example of an analog application where some gain and some “gain helper” were needed to accomplish our useful little application of amplifying a thermocouple probe.

Read More at Hackaday



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ESXi 5.0, 5.1, 5.5 Install stuck at loading module megaraid_sas

ESXi 5.0, 5.1, 5.5 Install stuck at loading module megaraid_sas

My install on an old Dell 1950 was stuck at “Loading module megaraid_sas” step and redoing the DVD drive as EFI did not fix it.  Since this was clearly a reference to the LSI raidf adapter which is also known as a Perc, I installed the ESXi 5.5 image from Dell complete with drivers and it worked on the first attempt.



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3D Object Convertor

I try to take note when I see a useful program in hopes that I can find it when the need actually arises.  I use Proteus ARES for my PCB and schematic and it has a cool 3D Viewer for the final PCB.  Authoring the objects is a different animal than just using the pre-existing 3D objects but the image mapped objects seem pretty sparse.

Check out the list of 660 file formats that they convert at: 660 File formats for 3D Conversion



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Searching Outlook Using Windows 8 Broken

I noticed that even simple things on my stupid Windows 8 that I got stuck with thanks to the aggressive addiction policies of Newegg/ASUS/Microsoft wouldn’t even properly search my Outlook contacts. Forget the cutesy interface I need my computer  to allow me to do business efficiently. Don’t get me going on charms and having to move the mouse and click repeatedly to do what a simple start menu did. (I have installed a 3rd party start menu)

The fix was simple but just one more tedious step in what shouldn’t be broken to begin with.

The fix in my case consisted of opening Outlook, going to File>Options>Search>Indexing Options and removing Outlook from the index.

Then close Outlook and make sure it completely closes by waiting or checking with taskmanager.

Open Outlook and re-add Outlook to the Index.


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Getting Windows 8 to work with Cisco VPN Client

I got stuck with Windows 8 pre-installed, lets just say I wont be buying from NewEgg anymore sadly.  Efforts to install Windows 7 even after negotiating UEFI failed, I believe that the BIOS has been specifically munged to thwart the 7 install. Asus simply says you cant go back.

So why I hate Windows 8 starts with the fact that I am a business/tech user, I don’t need to draw pictures for my mother or swipey swipey with my finger.  I need VPN’s to work and ASDM software to work.

Tip #1:  How to get Cisco VPN Client to work with Windows 8
Open Registry editor by typing regedit in CMD prompt
Browse to the Registry Key  HKEY_LOCAL_MACHINE\\SYSTEM\\CurrentControlSet\\Services\\CVirtA
Select the DisplayName to modify, and delete the leading characters in front of “Cisco”

For x64, change the value data from something like “@oem8.inf,%CVirtA_Desc%;Cisco Systems VPN Adapter for 64-bit Windows” to “Cisco Systems VPN Adapter for 64-bit Windows”


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VCF Talk with Bil Herd: Working for Commodore

Bil’s talk at VCF in May of 2012


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Cisco ASA 8.4 – No more Global Address Pools

There are a couple of ways to do NAT/PAT assignments as you might expect out of 8.4.  Assuming that you don’t really have a single Net Object that represents the entire inside network I recommend not using the Net-Object method and to define a rule “outside” of the Net-Object framework.

So first define a network object for NAT range of external IPs and then a PA  external IP address.  In cli it looks like this, these are external IPs just to be clear:

object network nat-range1

object network pat-ip1

You can do the same easily from the ASDM but I wanted to make sure the size of the block as a range instead of a subnet was visible.


Now from the NAT page create a new Dynamic  Rule,


The NAT Pool should look like this when done, I use inside to Outside2 here.


Note the nat-range object which used to be a “pool”.

Now add a PAT.  It cant overlap with the NAT pool, etc etc.  Don’t choose Round Robin as it’s memory intensive. I believe I read that 8.4 has an issue where it can run out of certain types of PAT ports (they try and group all ports below 1024 together, etc) that from what I gather is fixed in 8.51 <sigh>

Add Nat Rule after "Network Object" NAT Rules

Add Nat Rule after "Network Object" NAT Rules

Should look like this when done, I moved it to the top for clarity.


I recommend making this change separate from other work so it can be tested separately, TEST for a couple of hours make sure it is NATting and patting correctly under load is my advice.


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Getting Cisco Netflow to work on Solarwinds

I don’t have the details as to which version of IOS does what, we just call it the “new netflow” where you separately specify the exporter, the monitor, etc.  It’s usefulness comes from it’s ability to be specific and puts to bed once and for all the whole “is it version 5 or 9 Netflow?” question since you have control over it.

Here is a basic setup that uses UDP 2055 for Solarwinds instead of the standard 9991.

flow record NF-Record1
     match ipv4 protocol
     match ipv4 source address
     match ipv4 destination address
     match transport source-port
     match transport destination-port
     match interface input
     collect transport tcp flags
     collect routing forwarding-status
     collect interface output
     collect counter packets long
     collect counter bytes long
     collect timestamp sys-uptime first
     collect timestamp sys-uptime last
flow exporter NF-Export1
     destination aaa.bbb.ccc.ddd   <-- IP Address of collector/Solarwinds system
     source Loopback0
     transport udp 2055
     template data timeout 60
flow monitor NF-Monitor1
     record NF-Record1
     exporter NF-Export1
     cache timeout inactive 30
     cache timeout active 60
     cache entries 1000

interface Gig x/xxx   <-- the interface to be monitored
  ip flow monitor NF-Monitor1 input

I added additional collection stats out of habit in the flow record, it’s worth investigating what other flags/packets your interested in.

To view the exporter settings do a

show flow exporter
4500-Switch#sho flow exporter 
Flow Exporter NF-Export1:
  Description:              User defined
  Export protocol:          NetFlow Version 9
  Transport Configuration:
    Destination IP address: aaa.bbb.ccc.ddd
    Source IP address:      eee.fff.ggg.hhh
    Source Interface:       Loopback0
    Transport Protocol:     UDP
    Destination Port:       2055
    Source Port:            56118
    DSCP:                   0x0
    TTL:                    255
    Output Features:        Not Used

To switch between Netflow Version 5 and 9, issue the export-protocol netflow-v5 or netflow-v9 from within the exporter config as see below as command completion:

4500-Switch(config)#flow exporter NF-Export1
4500-Switch(config-flow-exporter)#export-protocol ?
  netflow-v5  NetFlow Version 5
  netflow-v9  NetFlow Version 9


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