Some Keithley femtoamp-grade toys arrived:
Great part about these little cute amplifiers is the fact that they are bidirectional for current. This is especially useful to use together with SMU.
If we use with SMU that have lowest range 1uA (such as Keithley 2400) amplifier can be configured as 106 attenuator, while providing accurate feedback signal for SMU to provide regulated loop. In this configuration Preamp will be configured such as block diagram below.
If user want to measure very low current, such as 1 pA, PA will be configured as gain block with 106 amplification, tracking tiny input currents with guarding. For such operation direction will be “reversed”:
Little bit of careful poking with soldering iron to remove shield:
Aha, shields marked as Keithley 6430 preamp :)
How we can make better sense of block diagram from US6069484 patent.
It’s a lot of stuff happening in here. No wonder it’s so packed so dense.
TMS370C702CWT chip in PLCC socket in small 8-bit microcontroller.
There are total no less than TEN special TI LMC662 2fA bias opamps.
Reverse engineering of Keithley 4200 preamp head
First we need to remove ceramic resistor networks to gain access on top side for probing and photoshoot:
Assuming pin 1 is marked with dashes measurement results of CN-343-1 resistor network are:
|Pin 8 – 9||19.81 KΩ|
|Pin 1 – 8||not connected|
|Pin 1 – 7||200.6 KΩ|
|Pin 1 – 6||2 MΩ|
|Pin 1 – 5||20 MΩ|
|Pin 1 – 4||200 MΩ|
|Pin 1 – 3||2 GΩ|
|Pin 1 – 2||20 GΩ|
Resistors are manufactured by Ohmcraft. CN-342-1 was not giving any meaningful readings. Perhaps it’s broken?
Pinout for control HD15 port
|4||Logic +5V to CPU/I2C EEPROM|
Keithley 6430 preamp head
Shortly after this project was started, opportunity to grab actual 6430 PA revealed itself, and we did not hesitate.
Quick check with Keithley 6430 unit, to confirm that module is indeed operational:
Now we can take it apart, to compare with Keithley 4200 PA.
As one can see, similar design by idea, but quite different implementation and indeed different schematics.
To be continued.
Keithley 6430 fA-SMU repair
Thanks to FF, xDevs.com also acquired broken 6430 unit (w/o PA), with diagnosis “no output”.
Received very well packed device, leaving no chance to easy damage of fragile instrument:
First smell check didn’t reveal any issue, so turning on confirmed display operation, responsive buttons and only factory calibration, with count = 1.
Check with DMM at the output indeed confirmed seller’s diagnosis, no output for either current or voltage. Four screws and 5 minutes later:
Analog PCB is based from Keithley 2400 SMU. Photo of K2400 analog PCB shown on next image to refresh memory.
Image X: Keithley 2400 analog board
There are additional modifications for external 6430 very low-current amplifier. These include isolated 1W DC-DC Murata/Power Convertibles HL01R05D05Z and extra optocouplers for SPI interface to 6430 preamplifier. DC-DC specified for 100mVpk-pk max noise in DC to 10MHz bandwidth and isolation capacitance just 25 pF.
Closed look revealed the problem:
Look closer in top right corner. See anything obvious? It can’t be only that, right? It would be too easy.
But yes, after restoring connection to analog board power supply input indeed made unit fully functional again.
Since we in here anyway, let’s remove analog board and see on the digital/power supply boards located under.
Keithley 6430 using special multicore planar transformer assembly to provide low-noise high-isolation power to analog board.
Standard Keithley 2400 does not have any of this fancy stuff.
Otherwise this is just usual Keithley SMU and it works exactly same way (minus the 1.05A current range) as 2400 when Keithley 6430 amplifier is not connected. There are front terminals available with 6430, so output/input provided at the rear ports only.
Modified: July 18, 2019, 1:55 p.m.