IC104, IC105, IC106 = OPA140 (from TI)
R116 =1R (1%) / 2512
R117 = R119 = R120 = R121 = R122 = R123 = 10K (0.1%) – 0603
R118 = 20K
R124 = R125 = 100K
Jig_Filtered_Supply = 9V
R280 = 1.5K
R281 = Not fitted
C207 = 1nF
D204 = D207 = 0.5uA leakage Schottkey Diode (Farnell-1459171)
IC215 = LTC2485 (24bit ADC)
It can not go exactely to the supply rails.Does this mean that with NO input applied at sense resistor (R116), I would have a non-zero output voltage due to intrinsic characteristics of any opamp?
This is quite straight forward:Which parameter on datasheet would define this?
Correct!does that mean I need to have at-least 375-400mV Reference voltage applied to the node of R122 which is currently connected to GND(0V)?
This offset is caused by the two 10k resistors across 1V. The current is 50uA. 50 uA on your 1 Ohms shunt causes 50uA.How did you calculate the offset of (1V *1 Ohm)/20k? There are plenty formuale in various text books but I learn and understand only when I actually hit a problem
No way.BUT would that allow me to measure the current accurately to 0.1mA resolution (ideal) without calibrating the jig for current measurement
Yes. You'll connect the R122 foot point to a positive reference voltage and substract 0.5 V (or whatever the offset at the final ADC is) when processing the measured data.Are we saying that by applying external reference voltage of 0.375 will put this circuit in a known controlled state? How?
You answered it by yourself. Yes.Now, regarding the reference voltage application to opamp to compensate for the output saturation voltage, is it true for Buffer circuit as well?
The buffer is designed using AD8603 which could have output saturation of upto 50mV (for load of upto 1mA)
Only one shift (by a reference voltage) is sufficent. It should be in front of the first AMP.Would I need a external ref greater than 50mV for buffer circuit as well?
Only one shift (by a reference voltage) is sufficent. It should be in front of the first AMP.
Klaus
Rail voltage | Reference(at R122 node to R_new2) | Output of IC106 (opamp) (no diode) |
0V | 351.2mV | 351.1mV |
1V | 356.4mV | 356.44mV |
2V | 361.7mV | 373.06mV |
3.5V | 369.8mV | 373.06mV |
4.2V | 373.6mV | 399.1mV |
Variable Supply Rail | Buffered Ref output | OPAMP-IC106 output | OPAMP-IC106 (+) input | OPAMP-IC106 (-) input |
0V | 429.7mV | 429.5mV | 219.3mV | 219.3mV |
1V | 429.6mV | 429.6mV | 714.4mV | 714.7mV |
2V | 429.6mV | 429.6mV | 1.2129V | 1.2134V |
3.5V | 429.6mV | 432.9mV | 1.9622V | 1.9631V |
4.2V | 429.6mV | 454.9mV | 2.32V | 2.321V |
Variable Supply Rail | Buffered Ref output | OPAMP-IC106 output | OPAMP-IC106 (+) input | OPAMP-IC106 (-) input |
0V | 814.5mV | 814.3mV | 411.7mV | 411.9mV |
1V | 814.4mV | 814.4mV | 906.7mV | 907.1mV |
2V | 814.5mV | 814.5mV | 1.4052V | 1.4058V |
3.5V | 814.5mV | 817.9mV | 2.1546V | 2.1555V |
4.2V | 814.5mV | 840.2mV | 2.5125V | 2.5137V |
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