LMC6001: Case CS0216207

Hi Fred,

Could you share the schematic with us? 

You may use the icon in red to post an image file. You may use the icon in green to post a std file over the forum. 

Best,

Raymond

Believe the schematic has been uploaded.

Thank you for your help.

Fritz LangDiesel Fuel Quality Schematic Rev07.pdf

Hi Fred,

I forgot to ask what types of waveform is 22uA signal as in put. Anyway, I put together something that might work for your application. Using LMC6001 op amp, I added BUF634 buffer at its output. BUF634 is rated up to 250mA. It should be good enough. If you have to driver slightly higher than 200mA, we can use LMH6321 buffer to drive the load. Please let us know what you prefer. The circuit has the BW at approx. 22kHz, see the Tina simulation. 

Based on your driving circuit, you have 12V rail available. The BUF634 buffer needs to have slightly higher rail voltage. It will work with 13V in simulation. 15V power rails will work for both LMC6001 and BUF634. 

/cfs-file/__key/communityserver-discussions-components-files/14/LMC6001-Integrator-_2B00_-BUF634-07222020.TSC

Best,

Raymond

Raymond:

The following are answers to your questions - as best as I can.

The 22uA signal's waveform is unknown; we are using an established vendor's signal which is: 1) sensitive to ion changes but not water (although the half live is in seconds, not minutes); 2) fully agreed that assuring steady state is critical (believe we have a handle on that); 3) it is not a water pH instrument per se, but is measuring resistance across sensitive electrodes.

Q: Should not be a problem to generate a 15 vDC rail ... BUT I understand you consider this critical ???!!!

Q: Thank you much for the schematic ... exactly that I was hoping for ... IF the 15v rail is critical, would you see changes in cap & resistor values ????   Initial market is targeting the pleasure boats, which run 12.3 to 13.5 vDC.  Do I need a circuit to assure a constant 15v, or could I set it at nominally, say, 16v and not worry about variation (say 15 to 16 v) ???  

Q: The LMC6001 spec sheet calls for an INPUT signal in the fA range ... I assume a  22uA signal is not too high of current ????

I CC our schematic .... you should of gotten it.

Q: You mentioned simulating these with software ... ould you point me in the right direction ????

Best Regards

Fred (Fritz) LANG

Raymond:

After studying your schematic, I have some questions:

A)  What are the tolerances for voltages V1, V3 & V4; can they vary uniformly ?? We are dealing initially with a pleasure boat application, voltages will range from 12.4 to 13.5 vDC.  How critical are V1, V3 & V4.  Could transform the rail source to, say 16 vDC (max), and then swing V1, V3 & V4 accordingly ??

B) For LMC6001's pin 3, hat is the 6.5 v do ??

C) What is BUF634's Bw ??

D) What is Vp ... test ???

E) LMC6001 input specification calls for a max fA - type signal; indeed will a uA signal work ?? 

Thank you

Fred (Fritz) Lang

Hi Fred,

Q: Should not be a problem to generate a 15 vDC rail ... BUT I understand you consider this critical ???!!!

The voltage regulation from standard LDOs will do. From the simulation, I see that the buffer will require at 13Vdc. I think that a typical LDO is accurate to +/-5% in its load regulation.  So 15Vdc rail will help for certain fluctuation of input voltage. 

Q: The LMC6001 spec sheet calls for an INPUT signal in the fA range ... I assume a  22uA signal is not too high of current ????

Per the absolute max. rating, I do not see that this is an issue, see p.3 of the datasheet. Is there a particular reason that you pick this one? The op amp is designed for high input impedance and TIA application. 

Q: You mentioned simulating these with software ... would you point me in the right direction ???? 

Please download the Tina simulation software. It is free. 

https://www.ti.com/tool/TINA-TI?keyMatch=TINA%20SIMULATION&tisearch=Search-EN-everything

Best,

Raymond

Hi Fred,

A)  What are the tolerances for voltages V1, V3 & V4; can they vary uniformly ?? We are dealing initially with a pleasure boat application, voltages will range from 12.4 to 13.5 vDC.  How critical are V1, V3 & V4.  Could transform the rail source to, say 16 vDC (max), and then swing V1, V3 & V4 accordingly ??

The absolute max. supply rating for LMC6001 is 16V, but BUF634 is rated up to  +/-18V (single rail can go up to 36Vdc).  If the input to LDO is 16V or so, and op amp voltage rail is around 13-15Vdc with 1-2% load regulation, it will work fine. 

B) For LMC6001's pin 3, hat is the 6.5 v do ??

C) What is BUF634's Bw ??

D) What is Vp ... test ???

That is my point test. I want to make sure that the LM6001's output is not distorted with feedback from BUF634's output. I saw the issue when BUF634's supply voltage rail is configured at 12Vdc. The Vp distortion went away, when I increase BUF634's supply rail to 13V or greater.  

E) LMC6001 input specification calls for a max fA - type signal; indeed will a uA signal work ?? 

I do not see issues. 

I have not played around the BW of the circuit. I think that the BW of the circuit may be able to increase it further. I will send you another version later. 

Best,

Raymond

Raymond:

Somewhat surprised by your questioning re using LMC6001 ???  

In my ignorance, I found the 6001 the only one to handle a low current .. could you suggest another.

Again the signal I need to integrate is relatively low mA, but its voltage (critical to our purposes)  varies from 3.5 to 10.0 vDC.

Thanks

Hi Fred,

I was wondering if you'd like to have a single IC solutions. For instance, OPA564 will work for you, but it is still slightly overkill for the application. ALM2402F-Q1 is dual op amp high current driver, that may work for your application. You may need to use only one op amp, but it is a low cost approach. Similar to ALM2402F-Q1, we have ALM2403 driver that has higher BW and current driving capability than ALM2402F-Q1. 

https://www.ti.com/lit/ds/sbos927a/sbos927a.pdf?ts=1595535615069&ref_url=https%253A%252F%252Fwww.google.com%252F

https://www.ti.com/lit/ds/symlink/opa564.pdf?ts=1595535216483&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FOPA564

We will have plenty two ICs driving solutions for this, but  it will cost more than a single Op Amp solution. Are you looking for high precision I-to-V conversion in your application? Since the input current is increase to sub-mA to mA, ALM2402F-Q1 may work for you. You may find some use for the second op amp or we can use it to drive as a dummy load or use it as a buffer. 

Please let me your choices, and I will put something together per your application. 

Best,

Raymond

Raymond:

Thank you.

Actually expense is NOT an issue.  The gizzo is intended to protect some very expensive engine parts; and must be reliable re human safety considerations.

So, if you think I can get higher precision re output voltage (i.e., the same as the input, or a constant scalable V), then dual op Amps would be interesting. 

We are quite willing to pay much for a 200-250+ mA output having the same V(in) ... with this are downstream circuit would be gold.   

In reviewing the specs, ALM2402F-Q1 appeals given its input voltage range.   

Thank you

Fritz Lang

Hi Fred,

ALM2402F-Q1 is used to drive resolver's excitation voltage in a vehicle. . 

Resolver is used to measure vehicle's shaft rotatory speed and may be used to calculate the engine torque at certain speed. The part is automotive grade and it is very rugged for the design application. A single Op Amp and low part counts are also plus for reliability reasons. 

Enclosed is TIA circuit from the IC. I assumed that constant input current of 2mA. Also, I check the square wave to verify the Op Amp stability. There are plenty phase margin in the circuit, and the cutoff frequency is measured at approx. 41kHz (it can configure higher, but I do not believe that you will need it for noise consideration). 

If you have additional questions, please let us know. 

/cfs-file/__key/communityserver-discussions-components-files/14/ALM2402F_2D00_Q1-TIA-07242020.TSC

Best,

Raymond