DAGAN HEADSTAGE PROBE
for Chem-Clamp patchclamp mV/nA
SYMPTOMS: with input connected to supply common (via 1000M resistor,) display shows overload.
The headstage is an LF356 op amp with an added input stage, using matched dual JFET wired as its front end (discrete diff-amp stage,) w/1.1K drain resistors to +10Vdc, with an NPN 2N3904 wired as a 4mA const-current driver on the common JFET source pins to -12Vdc. The axon main input goes to JFET's neg diff input, and to 100M feedback resistor, while the pos diff input goes to Dagan's active-driven shield.
Note that the typical full Dagan system gain (transimpedance) through the instrument is actually 10X higher
than just the headstage alone. The headstage 100Meg feedback resistor gives
a main instrument gain = 1V/nA, while the gain of the headstage itself is only 1V/10nA as expected from 100meg. (You could swap out the 100M resistor to customize the gain (and max/min clipping) for various signals, try 10M, or 1M, or 500M! )
Typically our JFET on the main input pin is fried, static-zapped. Ohmmeter shows significant leakage in both directions (wrt supply common, blue wire.)
It's a matched Dual JFET in TO-71 6-pin metal can, marked "0743-TM LS", but it's
actually a Linear Systems LSU401 or U401 dual JFET. Also sold by Mouser as InterFET IFN401, also Calogic U401 from Future Electronics, Siliconix, or see eBay. (U401 is selected for 5mV offset, U402 is similar but with 10mV offset, U403 has 10mV offset
but twice the temperature drift. U440 also exists, but w/bad ratings. Also see recent new part, Linear Systems LSK389 nacsemi.com)
Headstage cable DB15M Dsub 15pin connector:
3 - Green - Headstage Output (to controller)
4 - Gray - Driven shield (from controller)
7 - Clear, Blue - Main chassis gnd
8 - Red - pos 12VDC
9 - Black - neg 12VDC
120VAC Wall Power Supply, DIN-5 male conn
Pin 1 - Not Used
Pin 2 - + 15 Volts
Pin 3 - Not Used
Pin 4 - - 15 Volts
Pin 5 - Circuit Ground
Shell - (PS-2: Not Used), (PS-1: Chassis Ground)
