Can OPAMP unused pins left open?

Hi,

Currently i am using AD8544 for a test circuit to prototype a latching circuit using an Op-Amp. Here is the circuit

I have wired as per above. 

Setup: When Vsense is above than Vref, o/p goes high and D2 gets forward biased and this becomes a closed loop so even when Vsense is removed the output is high(LED will glow). 

Problem statement:

1. When Vref is turned ON first and then Vsense and then Vcc, the circuit works properly and when Vsense is above Vref LED remains ON. working properly.

2. Back to OFF state, I turn ON Vcc first(Vsense and Vref is not turned ON yet) the LED continues to be ON ie O/p goes high. 

I am not clearly getting what is the problem. 

AD8544 is a quad opamp. I am using only 1 of them. others are left open. will this create such a scenario?

Please help me.

Thanks in advance.

Parents
  • Hi, AAnand928

    Where the Vref pin is connected in second case? Way for flowing a bias current is required.

    Bias current applied to R4 resistor and cause drop voltage on this. This voltage applied to non-inverting input and amplifying. Output voltage becomes large then drop voltage on diode and circuit is latching

    Regards,

    Kirill

  • Hi KirV,

    In the second case, only Vcc has been applied. Vref and Vsense is floating. 

    Yesterday i did some more modifications. please see below diagram. I placed pull down for both inverting and non inverting pins.

    Applied only Vcc. Even now also LED is glowing.

    Regards,

    Ashok

  • Ok, you nave saturation of output stage but this is normal in this case. While the output voltage is below than drop voltage on diode, feedback loop is open and gain is too large. To inputs applied offset voltage and drop voltage on all resistances, connected between inputs and ground, caused bias currents. All this is amplified and output voltage begin increasing, positive feedback begin work and circuit is latched.

    500[V/mV] * 6[mV] = 3000 Volts! This is saturate. I'm sorry, but how you calculated this?

    You should limit the gain, adding resistor 1M of value between inverting input and output. Note, that R5 not required, because bias current will flow through 1M resistor.

  • Yes you got that correct. My intention is to latch the circuit in situations where non inverting voltage is higher that inverting voltage. 

  • 0
    •  Analog Employees 
    on Jul 18, 2019 4:38 PM over 1 year ago in reply to AAnand928

    Ok so it seems you are using this opamp as a comparator.

    First I want to get back to your first post, "When Vref is turned ON first and then Vsense and then Vcc, the circuit works properly and when Vsense is above Vref LED remains ON. working properly."

    Just want to point out that normally, you should do the proper power supply sequencing, supply first before the inputs unless you are doing something else I'm missing.

    For your problem no.2, you could try to put some hysteresis by adding resistor 1Meg in series with D2.

  • I am using this as a latch basically. When o/p goes high it should latch even if Vsense is removed.

    My query is that when I am turning on supply first even without giving high output goes to high.  

    My understanding was input offset voltage was too low like 0.1mV so there's no chance output can go high. Moreover i have pulled down both inputs. So technically speaking output should have been low. but its HIGH which is where i am getting confused. 

    Please correct me if am wrong

  • My understanding was input offset voltage was too low like 0.1mV

    Where did you get this value from? In datasheet of AD8541/8542/8544  says that offset voltage equal 1mV typically

    I have already said what needs to be done. You should limit gain so that your comparator does not react to offset voltage etc.

    On your pictures op-amp is AD8554 while you're talking about AD8544. What do you actually use?

    My circuit for you

    Regards,

    Kirill

Reply
  • My understanding was input offset voltage was too low like 0.1mV

    Where did you get this value from? In datasheet of AD8541/8542/8544  says that offset voltage equal 1mV typically

    I have already said what needs to be done. You should limit gain so that your comparator does not react to offset voltage etc.

    On your pictures op-amp is AD8554 while you're talking about AD8544. What do you actually use?

    My circuit for you

    Regards,

    Kirill

Children