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Original question:

INA114: INA114 or INA121 ?

LMC6062: t

CLAVERIE Alain
Intellectual 410 points
Part Number: LMC6062
Other Parts Discussed in Thread: OPA277, LM4040, LMC6061

Hi 

I asked a question to my orignial request

I've done the current generator seen on previous article based on a LMc6062 and reference voltage 

The current generator with the 91Khoms is about 27 uA

The INA 114 on the schematic is not assembled and a OPA277 in following mode is connected to the load resistor to measure the value

I've set up 5 generator on a card and I meet a strange problem

The 5 are totaly identical but

When I applie the power sometimes 2 of the 5 generator are not starting and deliver 0 UA

By puting power supply  on / OFF / ON several times the 2 failing generators becomes OK 

the load of the generator is between 100 K and 300 Kohms .. but the value seems not to disturb

Have you any idea .? 

thanks in advance

Regards

  • 0
    Intellectual 410 points

    Hi 

    I add some informations

    I deliver the 15V power supply throug a 2 contact relay '1 contact for gnd and 1 for +15V

    Sometimes the current generator start sometimes not 

    As if it was depending on the commutation of the relay 

    Another very stange problem

    When generator is not working only a simple scope probe NOT connected o r any wire . probe of multimeter not connected. a wire in fact or simply put my hand on the

    LM6062c and tje generator wil definitively start EACH TIME 

    I ve tried a lot of things with resistor to gnd , capacitor and so on .. no resultat

    Following is the chain I use .. aim is measure resistor betwee R+ an R-  (2 inputs on the upper left relay)

    Can you help

    Regards

  • 0 in reply to CLAVERIE Alain
    TI__Guru 60610 points

    Claverie, 

    The proper operation of the reference current circuit you implemented relies on the input voltage, Vin, and the value of R100 resistor.  For LMC6061 and LM4040 2.5V shunt voltage reference, the circuit should operate ONLY up to around 11V - see below.  

    You seem to be using Vin of 10V (close to maximum of 11V).  Since micro-power op amps, like LMV6061, may have a problem starting up in Howland current pump type configuration (with positve feedback), you may want to decrease Vin and/or increase R100 to assure the circuit properly comes up.

    LMC6062.TSC

  • 0 in reply to Marek Lis
    Intellectual 410 points

    Hi 

    first thank you so much for your answer

    I'm not sure well understanding sorry

    you talk about of VIn which is for me the voltage on the tested resistor

    so I dont understand how I can apply like you tell me

    it "s for me directly link to the current delivered by generator and except vari R100 I don"t see how I can

    any valu i put for resistor.. the pb still alives

    so ?

    thanks

  • 0 in reply to CLAVERIE Alain
    Guru 140200 points

    Bon jour Alain,

    take care, many relais cannot switch ultra small currents but need a minimum current. For switching 27µA you should choose a relay suited for so called "dry switching". This is a relais with gold covered contacts and which is filled with an inert gas.

    To see whether the relais contacts are the issue here, just remove the relais and have solder connections instead.

    Kai