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.
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|
Modified: Feb. 12, 2019, 12:33 p.m.