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I'am working with HB100 doppler sensor and I tried to build the amplifier shown in the datasheet of HB100. This amplifier uses opamp LM324, I don't know why it doesn't work. The Vout of the circuit is always at 2.5V and the Vout of the first opamp is alway on a fixed value. I have attached the schematic and layout of my circuit, someone could help me? What I'am doing wrong? I understand that LM324 is not the best way to amplify the IF signal of HB100, but at least I should see a signal at the output! Thanks.

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    \$\begingroup\$ What is the "fixed value" at the output of first amplifier? \$\endgroup\$
    – Ale..chenski
    Commented May 1, 2018 at 0:47
  • \$\begingroup\$ You shouldn't connect all unused pins to ground. TI recommends to connect only IN+ to ground, and connect IN- with OUT, and left it floating. \$\endgroup\$
    – Ale..chenski
    Commented May 1, 2018 at 1:07
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    \$\begingroup\$ @WhatRoughBeast, his gain is only 100, and it is on AC. On DC the gain is 1. I did look into this possibility, and found no apparent fault there. I am not sure if 2n2 caps in feedback make it stable though. \$\endgroup\$
    – Ale..chenski
    Commented May 1, 2018 at 1:41
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    \$\begingroup\$ @AliChen - Oops. Dang it, you're right. Silly me. I've deleted. \$\endgroup\$
    – WhatRoughBeast
    Commented May 1, 2018 at 2:06
  • \$\begingroup\$ The application note has for R8, R10, a value of 1m (m=milli) which is weird. There is also a caution about the device being sensitive to ESD in bold red. Could it be fried? \$\endgroup\$
    – HKOB
    Commented May 1, 2018 at 9:17

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The currents through polarized capacitors may drive the feedback current of 1 Million ohms to rail. And input bias currents thru 330Kohm may move stage1 Vout to be well away from VDD/2, putting a large bias voltage (of wrong polarity) across that DCblock cap between stages.

Do you need R5 to be 330 Kohms? make R5 10 or 100X smaller.

And what thermal noise floor would you like?

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  • \$\begingroup\$ Since the OP has 12k Ohm to ground on the input it would seem reasonable to reduce the 330k Ohm to around 20k Ohm. \$\endgroup\$
    – Jack Creasey
    Commented May 1, 2018 at 5:10
  • \$\begingroup\$ Hi @analogsystemsrf. I solved the problem with your tip, thanks! I saw this example scheme in the application note of HB100, why in the project is used this 330kOhm resistor if it takes problems? \$\endgroup\$
    – Fabio
    Commented May 2, 2018 at 7:09
  • \$\begingroup\$ FET opamps may not have such a problem. By the way, what random/Johnson/Boltzmann noise floor must you implement? What SNR do you need? and what upper F3dB can you tolerate? \$\endgroup\$
    – analogsystemsrf
    Commented May 3, 2018 at 4:42
  • \$\begingroup\$ I have implemented the circuit suggested in the HB100 application notes, there was this resistor of 330 kOhm and I followed this schematic. I must amplify the IF signal to read his frequency and manage it with a microcontroller, I need this part for a thesis project. I studied three opamps OPA2365, OPA2703 and OPA2350. The bandwidth of IF signal is no more than 1 or 2 kHz (for my scope). The best is OPA2365 for this scope, or not? Because it has the lower input voltage noise density and the best slew rate. \$\endgroup\$
    – Fabio
    Commented May 7, 2018 at 12:14

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