Keithley 2400 Test Points
         ---------------------------
         

The following describes some of the Test Points in the
Keithley 2400.  This text switches between the Digital Board
and the Analog Board without telling the user.  Some or much
of what is stated may be wrong.            16-Apr-2021


- check the Supply voltages first

- check that VFB and IFB (voltage and current measurement amplifier)
  corresponds with Vout and Iout as measured with a DMM.

- IF VFB is ok with negative voltage outputs, and the output buffer
  amplifier is ok, then the error could be in the range switching.

- check that the DAC output is ok

- check connectors


Note: The complete analog circuitry is floating and
voltages are referenced to the floating ground.

The floating ground is on the output of the power amplifier.

(NO - there are separate analog  "Floating"  ground / power
return    and    analog  "Output"  ground / power return).
These "grounds" are at different potentials.


(The following refers to the Digital Board).

VD voltages are referenced to digital GND.

TP3 on the power supply PCB is the Digital GND

TP4 is the floating ground for the analog circuitry -
TP3 floats with the output voltage - DO NOT connect
with digital GND!
(Actually TP3 is the Chassis Ground - TP4 is the
"Floating" ground - correct do not ever connect).

TP2 is +12VD Digital

TP1 is +5VD Digital

TP6/TP7 is +15VF and -15VF (floating)

TP8/TP9 is -30VF and +30VF (floating)


(Ther following refers to the Analog Board).

If you enter 1 Volt and 100 mA at the front keys, the
voltage and current DACs should change accordingly:

TP201 is the negated Voltage DAC control voltage

TP203 is the negated Current DAC control voltage


There is a special "Approximate Feedback" part.

TP204 is the negated control voltage for this part.

There is a "Polarity" voltage on TP202

The 2400 regulates the difference between the control
voltage (current) and the measured voltage (current).

There is a lot of switching for the different quadrants
depending if the unit is in voltage or current mode.

Test voltage and current mode independently with
No-Load or a small resistor at the output.

The result of this magic switching is on TP213, which
is the error drive going to the output buffer stage.

The main buffer drive is on TP214

The switching signals are on TP205-TP212

The output voltage is measured with a differential amplifier.

The input of the differential amplifier is SENSE_H and
SENSE_LO or a local version which comes from internal nodes.

TP227 is the buffered SENSE_HI input

TP228 is the buffered SENSE_LO input after the voltage range divider

TP230 ist the VFB (voltage feedback signal), the difference
of TP227 and TP228

TP231 is the CFB (current feedback signal).

TP233 is the ADC input signal, after the multiplexer which
switches between VFB and CFB.


There is a special supply +5VFL on TP218 and -5VFL on TP219
for the SENSE_LO voltage buffer (not floating)

(Actually TP218 and TP219 are referenced to the Output
Ground TP501   -   not with respect to the Floating Ground).


If you measure +-1 Volt between the input pins with a DMM and
measure a current of +-100mA through a load resistor, the VFB
and CFB voltage should correspond to the DMM voltage and
current measurement, scaled by the voltage and current range.

ISENSE_HI ( is at TP221

TP220 is ISENSE_LO (=output of power amp after current
sense resistors), which is buffered at TP222

The difference is the current feedback signal IFB

There is a current guard voltage on TP224

And some parts have a driven guard supply which floats in TP223