OPA2277-EP: Voltage seems can't be forced with positive input

user5802412

Intellectual 300 points

Part Number: OPA2277-EP
Other Parts Discussed in Thread: DAC9881, OPA2277, DAC9881EVM, REF5050

Hello,

I am using OPA2277-EP as kelvin connection for DAC's reference voltage such as Fig.1 .

( Fig.1: OPA2277-EP test circuit ---- referenced from SLAU279A p20 )

Firstly, OPA227-EP is operated with +/- 14.5V and "positive input A" is about 5V.

My cognition is that

1.Due to kelvin connection's purpose, VREFH-F1 should be forced to 5V

2.If this circuit work normally, the current consumption should be about 800 μA per amplifier ( fortunately when I measure the current from OP_SENS_V+ , It was about 1580 μA )

However when I power up this circuit, I found that

1. VREFH-F1 can't be forced to 5V but 5.6V and finally went up slowly to 13.56 ~ 13.58 V

2. VREFL-F1 can be about 0V ( this result is ok, seems the "positive input B" is connected to AVSS )

Whether there is something wrong about Fig.1 or my cognition ?

Thank you for any suggestion and help.

Wen

over 5 years ago

0 kai klaas69 over 5 years ago

Guru 140200 points

Hi Wen,

what DAC are you suing? Can you show a schematic including this DAC?

Kai

0 user5802412 over 5 years ago in reply to kai klaas69

Intellectual 300 points

Hello Kai,

Thank you for your reply.

I expect to use DAC9881 as my DAC.

However, I haven't integrate DAC9881 in my schematic..

Since before integrate DAC9881, I hope to ensure that the reference voltage for DAC9881 can be stable and precisely.

More precisely, my current test circuit has not been loaded with any load other than the voltage meter ( 34465A ).

BR
Wen

0 kai klaas69 over 5 years ago in reply to user5802412

Guru 140200 points

Hi Wen,

you haven't connected the above circuit to the DAC9881 yet?

Kai

0 user5802412 over 5 years ago in reply to kai klaas69

Intellectual 300 points

Hello Kai,

Yes, I let the VREFH_F1 ( J83 ) floating in Fig.1

but I have tried to connect J83 to a LDO as loading. It still can't be stable as its "positive input A"..

BR
Wen

0 kai klaas69 over 5 years ago in reply to user5802412

Guru 140200 points

Hi Wen,

your circuit will not work, if it is not connected to the DAC9881, because the feedback loops of OPA2277 aren't closed.

Kai

0 user5802412 over 5 years ago in reply to kai klaas69

Intellectual 300 points

Hello Kai,

Thank you for your reply.

>> because the feedback loops of OPA2277 aren't closed.

1. May I say since 2.2 nF ( BC58 ) connected between OUTA1 and "negative input A" which lead the feedback loop aren't closed ?

2. What are BC57, BC58 and R26 using for DAC9881's reference force pin ( VREFH of DAC9881 ) ?

3. If I want this feedback loop can be closed, I just let the DAC9881 in my test circuit in the future ?

BR
Wen

0 kai klaas69 over 5 years ago in reply to user5802412

Guru 140200 points

Hi Wen,

please have a look at figure 64 and figure 67 of datasheet ot DAC9881 to see what I mean.

The circuit in figure 67, which is almost identical with yours, can only work, when it is connected to the DAC9881.

Kai

0 user5802412 over 5 years ago in reply to kai klaas69

Intellectual 300 points

Hello Kai,

Thank you for your reply.

More question about figure 67 of DAC9881's Datasheet ,

1. Why use 2.2 nF connected between OUTA & Negative input A ? Whether 2.2 nF is used to reduce ripple of VREF ?

2. Whether 50 ohm and 1 nF work as low pass filter or another function ?

3. Since the feedback loop was not closed, but the quiescent current was about 1580 μA. Can I say that this OPA2277 is still alive or it have been damaged ?

Thank you!

BR
Wen

0 kai klaas69 over 5 years ago in reply to user5802412

Guru 140200 points

Hi Wen,

the 1nF cap works as a reservoir for fast transient currents drawn by the DAC9881. The 50R resistor isolates the output of OPA2277 from this capacitive load. And the 2.2nF cap in the feedback path acts as a phase lead capacitance to recover the phase margin of OPA2277.

Kai

0 user5802412 over 5 years ago in reply to kai klaas69

Intellectual 300 points

Hello Kai,

Thank you for your detailed reply.

I'll rearrange my schematic with DAC9881.

Could you recommend some reference voltage source for DAC9881 ( 1.6v ~ 5.5v ) ?

Finally, I try to make the conclusion for this topic.

1. The reason why I get about 13.56V on my VREFH_F1 ( J83 ) is due to the feedback loop didn't closed. The datasheet of OPA2277 have showed that when testing open loop gain , Vo may equal to ( V- ) + 0.5 ~ ( V+ ) -1.2V.

2. This test circuit only can work normally when connected with DAC9881.

BR
Wen

0 kai klaas69 over 5 years ago in reply to user5802412

Guru 140200 points

Hi Wen,

the DAC9881EVM uses the REF5050:

www.ti.com/.../dac9881evm

Your conclusions are correct.

Kai

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OPA2277-EP: Voltage seems can't be forced with positive input