This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

  • Resolved

OPA4188: Input Current in Chopper Op-Amps

Expert 2140 points

Replies: 5

Views: 164

Part Number: OPA4188

I am seeing current flow from the inputs of my OPA4188 op amp.  It usually happens if one op-amp output in the quad package is railed.  I thought this was a limitation of this type of op-amp.

Now I'm seeing 200uA coming out of the non-inverting node (pin 3).  No amps in the package have inputs near the rails.  The only output that railed is op-amp A, the one with 200uA at pin 3.  Supply is 5V and GND.  Pin 2 is at 2.21V.  Pin 3 is at 1.45V.  So the output is rightly at 0.  But what could be causing the input current?  

  • Hi Charles,

    Please provide us the schematic of your OPA4188 circuit. It is difficult for us to fully assess what you are observing without it.

    Thanks, Thomas

    Precision Amplifiers Applications Engineering

  • In reply to Thomas Kuehl:

    Thomas Kuehl

    Please provide us the schematic of your OPA4188 circuit. It is difficult for us to fully assess what you are observing without it

    I will post a schematic this afternoon, but I'm not sure it will help with my question.  The question is if 200uA current from the non-inverting node is normal for this amp and if it's okay for this op amp to be railed, i.e. have inverting and non-inverting inputs at different voltages so that the output goes to the rail.  

    I will post excerpts later.  Thanks! 

  • In reply to Thomas Kuehl:

    Thomas Kuehl

    Please provide us the schematic of your OPA4188 circuit. It is difficult for us to fully assess what you are observing without it.

    Here is the op amp where I'm seeing 200uA out of the non-inverting input:

    V[pin 3] = 1.446V.  V on the other side of the 10k resistor = 1.217V.  There is no AC component.  It's clean DC.

    Here's the rest of the op amps:

    None of the amps are railed.  

    Can you think of any reason current would flow out of the non-inverting input?  

  • Charles Gervasi

    Part Number: OPA4188

    I'm seeing 200uA coming out of the non-inverting node (pin 3).  

    I replaced the OPA4188 with AD8604 and the input current went away.  

    I originally had AD8604 in the schematic, but I need the higher supply voltage and low drift of the OPA4188 elsewhere on the schematic, so I made them all OPA4188 to reduce BOM line item count.  

    Why do I see input current on OPA4188 but not AD8604?

  • In reply to Charles Gervasi:

    Hi Charles,

    I know what is going on. You are applying the OPA4188 amplifier section as a comparator, with a 2.21 V reference voltage applied to the inverting input. When you drive the non-inverting input to the 1.45 V level you forward bias one of the internal anti-parallel input protection diodes shown in the datasheet Figure 42 repeated below. The current flows from the reference applied to the OPA4188 inverting input, through the input protection diode and external series 10 k input resistor, to the 1.45 V input source. The path established just how much current flows.

    The OPA4188 was TI's first high-voltage chopper op amp, and newer op amps have since been introduced that do not incorporate the internal anti-parallel protection diodes. Newer protection techniques have been developed and are used instead. 

    If you wish to stay with a chopper The OPA4187 may be an option because it doesn't use the anti-parallel protection diodes across the inputs. However, do note that it operates at lower current than the OPA4188. Therefore it doesn't have the same level of noise, or bandwidth as the OPA4188. You can view view the OPA4187 datasheet here:

    http://www.ti.com/lit/ds/symlink/opa187.pdf

    There is another family of precision op amps that are not choppers and offer very high precision performance. They use a technique called e-trim™ to achieve very low offset and drift. They do not have the anti-parallel input protection diodes either. The OPA4191, OPA4192, OPA4196 and OPA4197 are variations within the product family. You can view the OPA4192 datasheet here:

    http://www.ti.com/lit/ds/symlink/opa2192.pdf

    Regards, Thomas 

    Precision Amplifiers Applications Engineering

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.