LM2904: LM2904 about the output voltage issue

Mickey Zhang

Genius 12080 points

Part Number: LM2904
Other Parts Discussed in Thread: LM2902, TS321, OPA2131

Hi Team,

The customer is using LM2904. VCC is 5V. The schematic is in the attach. The level 1 circuit is magnified by 19 times the differential circuit.

The level 2 circuit is magnified by 11 times phase circuit. When a 2V common voltage is both input the 2 pin and the 3 pin of LM2904, the output voltage of

U100A is about 0.6V and the output voltage of U100B is about 1.2V.

My understanding: For the level 1 differential circuit, if a 2V common voltage is both input the 2 pin and the 3 pin, since the offset voltage exists, the output

voltage is 3mV offset voltage *19 =57mV. This value is closed to 0.6V.

But for the level 2 circuit, 0.6V is the common voltage, the output voltage should be 0.6V*11=6.6V. Since VCC is 5V, so the output voltage cannot reach 6.6V.

I use TINA to simulate his circuit. But the output voltage of the level circuit is 98.57mV.

Q1: For the customer's output voltages, would you explain the reason?

Q2: Is my understanding correct? If my understanding correct, why is the test result different from the simulation result for the level 1

differential circuit?

Best Wishes,
Mickey Zhang
Asia Customer Support Center
Texas Instruments

over 7 years ago

0 Ron Michallick over 7 years ago

TI__Guru*** 149776 points

Mickey,

Ideally the output sink current for LM2904 should be less than 12uA to get low output voltage near ground. As drawn, 2V common mode requires 2V / 21k = 95uA. To get this current to be less than 12uA increase R4 to R7 by at least 8 times. For example 11k and 210k will work.

0 Mickey Zhang over 7 years ago in reply to Ron Michallick

TI__Genius 12080 points

Hi Ron,

Thanks for your reply.

Q1: I see some other op amps datasheet shows the output voltage range directly. But the LM2904 datasheet shows the VOH value and the VOL value of the output voltage. So I am confused.

Why does the LM2904 datasheet show the VOH value and the VOL value of the output voltage, not show the output voltage range directly?

Is the output voltage range from VOLmin to VOHmax for LM2904?

Q2: The LM2904 datasheet shows when VCC is 5V, VOHmax is 28V. Do this mean when VCC is 5V, the output voltage can reach 28V?

Q3: For your reply, why does the output voltage be needed to be near ground?

Q4: Why is the output sink current 2V/21K? Could you explain more about this? What is the current (Vn-Vout)/21k? the output sink current ? or the output source current? Vn=2V*R5/(R5+R4)

0 Ron Michallick over 7 years ago in reply to Mickey Zhang

TI__Guru*** 149776 points

MIckey,

A1: I'm not sure what is meant by output voltage range that doesn't use VOL , VOH terms. LM2902,LM2904,TS321 (and it's name variant) have a unique output stage not used in any other op-amp. Can you provide a example of a output voltage range expressed directly?

A2: The data sheet doesn't say that. for VCC = 5V, output can not be 28V

A3: In the first post, the expected output was 57mV. That is close to ground.

A4: Yes, (Vn-Vout)/21k is a more complete formula for feedback current from output pin.

0 Mickey Zhang over 7 years ago in reply to Ron Michallick

TI__Genius 12080 points

Hi Ron,

Q1: E.g, OPA2131. Its output voltage range is from (V+ - 3)V to (V+ - 2.5)V. Would you explain the the VOH ,VOL value and the output voltage range?

Q2: For LM2904, the datasheet shows VOH is 28V when VCC is 5V on page 5. Would you explain the value of VOH is 28V?

Q3: Would you explain why the output sink current is 2V/21K, not ( Vn-Vout)/21k? Could you explain more about this? I still do not understand this.

0 Ron Michallick over 7 years ago in reply to Mickey Zhang

TI__Guru*** 149776 points

Mickey,

I do like the clarity of the OPA2131. This is one of the advantages of data sheet from this century. LM2904 could be updated in this way, however it is a many supplier device. If just one supplier changed to a new format then new customers would see a difference and think there is an incompatibility.

This is OPA2131 VOH , VOL

This is LM2904 put into the same terms.

Sink current is (Vn-Vout) / 21k

0 Mickey Zhang over 7 years ago in reply to Ron Michallick

TI__Genius 12080 points

Hi Ron,

Thanks.
Q1: For LM2904 and OPA2131, I am still confused about the VOH, VOL. How to determine the output voltage for the LM2904 ?

E.g. For LM2904, if VCC is 5V, what is the output voltage range? If VCC is 26V, what is the output voltage range?

LM118 is given the output voltage range directly.

0 Ron Michallick over 7 years ago in reply to Mickey Zhang

TI__Guru*** 149776 points

Mickey,

I already answered those questions. Try the free videos that cover these questions here.
www.ti.com/.../precision-op-amps-precision-labs.page

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LM2904: LM2904 about the output voltage issue