LOG114EVM: Adding a photodiode intensity preamplifier to shift the range

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.

Dear Thomas, 

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 suggested 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.

Kai

Dear Thomas.

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.

Hello Maxime,

There are a couple of issues with circuit that I have found upon my first review. Please consider these items:

1. The photodiode connections need to be reversed. The Vbias voltage which will attempt to appear at the LMP7721 summing node will be instead forced to the forward bias voltage level of the photodiode. Even if the photodiode produces current it will be in the direction such that LMP7221 output voltage will try to swing below the output level established by Vref. If the diode connection is reversed the diode becomes reversed biased and the LMP7721 output will move positive in response to the generation of photodiode current.
2. The LMP7721 Vbias is set too low. Vbias as is establishes the LMP7721 output level at +1.25 V, which is too low relative to the LOG114 VCM_in voltage of +2.56 V. The LOG114 requires the input current flows into the input pin. With VCM_in higer than LMP7721 output voltage the current will attempt to flow in the wrong direction. The goal is to set Vbias to a level such that no current flows through the LOG114 input V-I resistance when the photodiode current is zero. Then, as the photodiode does produce current the LMP7721 output moves positive and sources current to a LOG114 input via the series output resistor.
3. Your schematic shows both a 100 Ohm Rout and 1 k Ohm Ri between the LMP7721 output, and the LOG114 input. Do you have a particular reason for using two resistors?

Regards, Thomas

Precision Amplifiers Applications Engineering

Hello Thomas;
1) Yes, I should reverse the photodiode's anode and cathode connection on the inverting input to have a current flowing towards the LMP7721

2) The Vref on the LMP side is the Vref fed on the board. You may notice the J3 and J6 connectors are shunted, thus, Vbias should equal 2.50V. The LMP7721 output voltage should at least overcome the Vbias of the LOG124EMV input (2V, eq). The VCM_in voltage of the EMV board is equal to Vbias delivered and regulated at 2V by the OPA365

3) Rout seems to be embeded on the LMP7721 board (100 O, figure 4) Keeping only Rout would drain too much current on both sides, adding Ri reduces this current in the linearity range of the LOG114.

Regards

Maxime

Hi Maxime,

I made some changes to the current amplifier and LOG114 to see if I could set it up such that everything functions electrically and within their linear ranges. The TINA TI Spice simulation circuit I show below looks like it might to be getting close to achieving what you need. 

The TIA gain resistor was reduced to 1 Meg in order to keep the LOG114 logout within its linear operating range. Also, there really isn't any reason to run the LOG114 input current beyond 1 mA for a full scale TIA input current of 1 uA. Doing so does away with the additional linearization circuitry when the LOG114 input current exceeds 1 mA. It simplifies things and can be accommodated with the LOG114 EVM board.

I didn't really know how you wanted to set up the two additional op amps A4 and A5 so I used the resistors you had in your diagram.

If you would like to run some easy simulations on the TINA TI Spice circuit, I have attached the simulation file for your use. You can obtain TINA TI for free from TI.com if you don't already have. Just do a search on TINA TI. The program is intuitive and easy to use; especially, when you have a ready built circuit.

Regards, Thomas

Precision Amplifiers Applications Engineering

LOG114_Cur_amp_02.TSC