Part Number: LOG114EVM
In the frame of developing a low-light sensing experimental board, I recently acquired a LOG114EVM development board. With included components and the reference current source locked at 500nA, the default range is stuck between 10nA and 10mA, for a floor output voltage Vout of 1.15V.
My application uses a low dark current photodiode VTB1013BH, sensitive in the green region (0.3A/W) in photovoltaic mode. Dark current is at most 20pA while full illumination outputs 1uA (5 decades). To obtain a full response from the LOG114EVM board, I need a current amplification of 1000. Precision low bias current OPA2107 and 1% resistors are available, what kind of circuitry do I need to feed the LOG114EVM a 1000 scaled current from the photodiode in photovoltaic mode?
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In reply to kai klaas69:
Thank you for your reply.
A TIA with high resistance providing U=Id x R is indeed a good solution. If a resistor is set after this adaptation stage, an increased current may be observed.
Concerning the input voltage offset, refering to this application note of the EVM, it looks like an independant source may be used on the «high current correction input» pins to fake a photodiode:
I wonder how to connect the TIA output though, and given the «high current correction» is substential above 1mA, using the other connecting configuration would lead to:
If I replace Vref by the output voltage Vs of the TIA, in the following configuration:
Could it work? Here I = Vs/Ri, where Vs = Id x Rt. The OPA2107 datasheet displays a 10pA bias current, half the value of the dark current of the photodiode.
I was also considering a single transistor as current amplifier.
In reply to Maxime Lamotte:
In reply to Thomas Kuehl:
Thank you for your advices, considering the LMP7721 EVM board as a TIA for the LOG114 seems appealing, but several points need to be cleared:1) If a gain resistance Rf is taken at 1M, LMP7721 output voltage Vs should range between 20uV to 1V, well enough to be discriminated by further circuitry.2) The log114EVM board embeddeds a level shifter polarizing Vbias (2V) on the non-inverting inputs: this implies the current will flow backwards if refering to eq 4.3) If I add a precision summing circuit (with OPA2107), adding Vbias to my LMP7721 output Vs, a resistor of 100 Ohms converts Vs+2V into I1, then the current should flow towards the LOG114 in the designed range (200nA to 10mA)
I cannot discard the Vbias on the LOG114 board as the power supply is asymetric.
Hi Maxime, See my responses yo your questions: 1) If a gain resistance Rf is taken at 1M, LMP7721 output voltage Vs should range between 20uV to 1V, well enough to be discriminated by further circuitry. I would try and maximize the TIA output voltage swing range for the 20 pA to 1 uA input current. The transimpedance gain could easily be increased to almost 5 M (V/A) and then the output swing range swing would be closer to 0 to 5 V. Yu would be able to utilize more of the op amp's dynamic range. No op amp has an output that swings exactly to the output rails so you have to factor in reasonable lower and output levels that are within the linear output swing range. The lighter the load you place on the op amp the closer it will swing to the rails. Be sure you understand which direction the TIA output will swing in relation to the photodiode current flow direction. If the input current flow is into the op amp circuit, and then through the gain set resistor to the output, the output will move negative. If you use a single positive supply the op amp output will not be able to swing below 0 V. If the diode is connected reverse from this the current will flow from the output, through the gain set resistor, and through the photodiode to ground. The output will move positive in that case. 2) The log114EVM board embeddeds a level shifter polarizing Vbias (2V) on the non-inverting inputs: this implies the current will flow backwards if refering to eq 4. The EVM is set up for a single +5 V supply and the +2 V Vbias would be a problem; the LOG114 input current must flow into I1, 0r I2, and cannot be reversed without adding a current reversing circuit. That adds more complexity which I think you would want to avoid. If you plan to use the LOG114 EVM the TIA output would have to be established with a minimum voltage of 2 V. Then, the output could rise upward from there. Unless you can get enough of A TIA output range from 2V to almost 5 V, it would be necessary to use an op amp that can swing higher than 5 V. The LMC6001 Ultra, Ultra-Low Input Current Amplifier is usable with a supply up to +15.5 V, but you wouldn't need to use a supply to be set that high. 3) If I add a precision summing circuit (with OPA2107), adding Vbias to my LMP7721 output Vs, a resistor of 100 Ohms converts Vs+2V into I1, then the current should flow towards the LOG114 in the designed range (200nA to 10mA) If you could show us how you intend to connect the photodiode to the TIA that would be helpful for discussions. My thought was to apply the +2 V to the TIA non-inverting input which would rise the TIA output by +2 V. Doing so will add some reverse bias to the photodiode reducing its junction capacitance and dark current. Regards, Thomas Precision Amplifiers Applications Engineering
Hi Maxime, as I said earlier I would modify the circuit a bit. I would use a dual supply voltage of +/-5V for the LOG114. This would ease many things. Then you could use the offset adjust scheme I sugested in this thread: e2e.ti.com/.../2713011 Also, the TIA should see a much higher supply voltage. The higher the feedback resistor can be chosen the lower the offset voltage issues are. That's why I recommended a 10M feedback resistor (Rt in your scheme). It transforms your photodiode current range of 20pA...1µA into the voltage range of 200µV...10V. To be able to handle this full range the total offset voltage must be smaller than 200µV. But the LOG114 has an offset voltage of up to +/-4mV. That's why I suggested the offset adjust scheme. (Keep also an eye on the offset voltage of TIA.) The voltage to current converting resistor (Ri in your scheme) can be set to 10k then. This converts back the voltage range of 200µV...10V into an LOG114 input current range of 20nA...1mA.
You can choose a supply voltage for the TIA which is higher than the supply voltage of LOG114. The 10k current converting resistor (Ri) should provide enough protection for the LOG114. An alternative is to protect the input of LOG114 by an additional protection scheme as discussed in section 7.3.7 of datasheet of OPA627. But I think an additional protection scheme shouldn't be necessary.
Thank you very much for your involvement.The photodiode D1 in photovoltaic mode connected on a short BNC cable is grounded on the cathode.Biasing the LMP7721 non-inverting input in TIA configuration would indeed ensure an always positive input on the I1 LOG114EMV side.If the LMP7721 evaluation board has Vref as Vcc (+5v), the Vbias obtained across the voltage divider R1 and R2 should equal 2.5V.Given a 2M Rf gain resistor, the TIA should output 2.50004 ... 4.5V, well inside the rails.
On the LOG114EVM side, the input voltage should drop by Vbias(2V) due to the level shifter, still usable. The selected voltage-to-current precision resistor Ri of 1K will feed the 4'th LOG114 pin a current between 500uA ... 2.5mA.
Please find enclosed a schematic of the slightly modified evaluation boards as intended to be used.
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