LMP91000: LMP91000 operation using an external resistance between C1 & C2 pins

Robert Alfonsi

Part Number: LMP91000

I am using the LMP91000 in 3 lead amperometric mode. After trying out using the internal TIA resistance values and verifying the gains, I tried out TIACN = 0x00 and with an external resistance of 330kohm.

I was quite surprised that the output changed quite a lot from the case with TIACN=0x1C (350kohm internal resistance). The big surprise was that pin C2, which looks like it is connected to the output of the TIA, comes up with quite a different voltage to the Vout pin. I thought these two should follow one another?

Are there any circumstances in which the voltages on C2 & Vout can separate, other than temp measurement mode which I am not using?

over 6 years ago

0 Jalen Tate89 over 6 years ago

TI__Genius 9675 points

Hi Robert,

I will get back with you shortly to answer your question.

Jalen

0 Jalen Tate89 over 6 years ago

TI__Genius 9675 points

Robert,

Sorry for the delay.

What difference between the voltage at Vout pin and C2 do you see? Can you share a scope shot?

Which mode are you using? Can you also share your register map with me?

Jalen

0 Robert Alfonsi over 6 years ago in reply to Jalen Tate89

Prodigy 80 points

Hi Jalen,

, the registers are setup as follows:

REFCN: 0x54
TIACN: 0x00
MODECN:0x03
LOCK: 0

Sorry I don't have a scope shot but measurements as follows:

C1: 2.23 V (Vref=3.3V)
C2: 2.17V
Vout; 2.74V

If I switch TIACN to 0x1C then C1 & C2 stay the same, but Vout drops to 2.24V.

regards

Robert

0 Robert Alfonsi over 6 years ago in reply to Robert Alfonsi

Prodigy 80 points

Hi,
Repeated the above procedure on another board and saw some a difference in the C2 pin voltage, so believe there may be a board connection problem (component on pad not properly connected to C2) behind this.

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LMP91000: LMP91000 operation using an external resistance between C1 & C2 pins