Cant seem to get ac noninverting opamp to work

Using Arduino General Electronics
1

I'm currently working on making a small signal generator. I use a Arduino UNO together with AD9833 break out board. It generates a nice sinus Vpp 600mV. Its all working as it should. The frequency range is 0Hz to12.5MHz.

What I have a issue with is the opamp. I have a LM385N wich I think should work fine, but I dont seem to get any output at all ????

For the supply I am using a 9 volt battery ( like the one from the smoke detectors).

I have it hooked up like this :
circut schematic

I use
C = 0.1uF
R1 = 10KOhm
R2 = 1KOhm
R3 = 10KOhm

witch I thought would give me an Gain of 11

I apperantly am doing something wrong sinece I dont get anything on the output, does anybody have any suggestions?

2

First rule on using opamps:
The + and - inputs always have the same voltage (no difference).

If your + input is on Gnd (0V) then the output also must stay at 0V to produce 0V on - input.

Extend R1 into a voltage divider providing Vcc/2/gain to the + input. Then the output will stay at Vcc/2 with no input signal.

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3

Is this what you mean ?
circut design

4

You probably mean LM358, which would be OK for low frequency audio signals.

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5

You got it :slight_smile:

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6

Thank you both i am looking forward to testing this afternoon. :slight_smile: :slightly_smiling_face:

7

So this is looking much better than before, I still dont get the output I would expect. But by I guess that is just tuning in R3.

Test circut

Osiloscope result :slight_smile:

8

This part of the circuit looks odd :thinking:

image
image438×663 95.6 KB

Example:

image
image1817×1324 120 KB

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9

Your output will always be at least 2V below the positive supply.

Ypu have effectivly split the supply from your 9V battery; +/- 4.5V. So the positive swing will only be about 4.5-2= 2.5V which is about what you see. Date sheet extract below.
image

To overcome this you could reduce the gain, or bias the input to 9V - 2V divided by 2 = 3.5V. Or use an amplifier that swings clsoer to the rails.

Your LM358 will not extend to the 125MHz you sadi you wanted in your original question. You need a much faster op-amp. Gain of 11 and 12.5MHz. GBW >>137.5MHz.

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10

You probably mean LM358, which would be OK for low frequency audio signal

Most definitely.
There is NO WAY a circuit designed for 20Hz to 20,000 hz (audio) should work with
a 12.5Mhz input signal.

I actually don't know what the optimum series reactance should be for the cap (I'm sure Mark does) but as an example I took 100 ohms as the target and solved for C
using:
Xc = 1/2PifC
Let f = 12.5Mhz (12.5E+6)
Let Xc = 100 ohms,
Xc = 1/2PifC => Xc = 100 = 1/2 x 3.14159265359 x 12.5E+6 x C
.'. C = 1/(100 x 2 x 3.14159265359 x 12.5E+6 )
C = 127.32pF (1.27323954473E-10)
Using my HP-48 Calculator I bought for my DeVry BSEET course in 1992,
Equation Solver : EQ = Xc = 1/2 x Pi x f x C;
Let C = 0.1uF,
Let f = 12.5Mhz
.'. Xc = 0.1273 Ohms
Let C = 10uF;
.'. Xc = 1.2732 mOhms

As frequency increases Xc reactance decreases.

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11

yes, that was me just borrowing a metal leg, not connected to the circut :slight_smile:

12

Thank, I did not know that. Then it makes much more sense :slight_smile:

13

A 10uF electrolytic cap will not be any good for 12MHz.

I meant to mention that before. :wink:

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14

You should try THIS op amp.

LT1816 OP AMP

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15

Thank you I was doing research based on your comment. Very nice of you :slight_smile:

16

Do you know WHY I recommended that op amp ? (hint: look at the SPEC)

220MHz Gain-Bandwidth Product
1500V/μs Slew Rate
6.5mA Supply Current per Amplifi er
Programmable Current Option
6nV/√Hz Input Noise Voltage
Unity-Gain Stable
1.5mV Maximum Input Offset Voltage
8μA Maximum Input Bias Current
800nA Maximum Input Offset Current
50mA Minimum Output Current, VOUT = ±3V
±3.5V Minimum Input CMR, VS = ±5V
Specified at ±5V, Single 5V Supplies
Operating Temperature Range: –40°C to 85°C
Space Saving MSOP and SSOP Packages

IMPORTANT NOTE : !
READ THIS !
Total Supply Voltage (V+ to V–) ............................. 12.6V
**Differential Input Voltage **
(Transient Only, Note 2) ........................................... ±6V

This means the TOTAL voltage drop allowed is 12Vdc.
That means you can run any combination of +/- supply
voltages (like +/- 6Vdc, +/- 5V, +8/-4, +9/-3 etc. ) as long as
the total P.S. voltage drop across the IC does not exceed 12Vdc.)

Robert

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