Standalone Differential Front-end

[jlsilicon]

Is there Test results as a working Metal Detector ?
- Coin Size , Range from Coil, etc tests ?

[KingJL]

Good work, thank you. What mean your comments in schematic "(match HH2 R12)" next to R2 resistor?

I look at the HH2 design as a starting point. Some things I change (see my HH2 build thread). In his case, I changed R12 of the HH2 from a 1 K to a 2K2. I did this to lower the lower the current during the flyback and the resultant effect on the decay whilst the flyback is above the diode threshold.

If your HH2 R12 is 1K, then R2 of the daughter board needs to be 1K.

[WM6]

Good work, thank you. What mean your comments in schematic "(match HH2 R12)" next to R2 resistor?

[KingJL]

I do not recommend building this board as presented above. Stage 2 (IC2) oscillates! I need to breadboard this to investigate, but I believe this is caused by a too low source impedance for stage 2. I believe that adding a 2.2K between pin 6 of IC1 and pin 2 of IC2 and another 2.2k between pin 3 of IC2 and GND will cure this problem.

Ok I have made the changes to the schematic and board layout. I decided to go ahead and etch a board to make the testing easier and to eliminate any circuit instabilities introduced by the breadboard kludge that I created previously. Here is a picture of the board before drilling. I also included the updated schematic, board, and PDF board masks into a new zip for version 2.

[KingJL]

Circuit Malfunction

I do not recommend building this board as presented above. Stage 2 (IC2) oscillates! I need to breadboard this to investigate, but I believe this is caused by a too low source impedance for stage 2. I believe that adding a 2.2K between pin 6 of IC1 and pin 2 of IC2 and another 2.2k between pin 3 of IC2 and GND will cure this problem.

[WM6]

I see that I forgot to mention that the daughter board plugs into the IC6 socket on my HH2 board!

I don't forgot to thank you for this tip.

[KingJL]

Instead I cobbed together a daughter-board that includes the differential pre-amp and the HH2 backend interface.

I see that I forgot to mention that the daughter board plugs into the IC6 socket on my HH2 board!

[KingJL]

HH2 Daughter-Board

I decided not to wait until I can etch a board for my separate differential FE (TX & differential pre-amp). Instead I cobbed together a daughter-board that includes the differential pre-amp and the HH2 backend interface. I used the INA217 as the differential amp IC’s as these are what I have available (and are easily obtainable as samples from TI). This will give me a platform to test different configurations using a differential FE.

The gain of the pre-amp differential amp (IC1) is set to 67. The HH2 back-end interface (IC2) is also a differential amp with a variable gain of 1.9 – 31, resulting in an overall variable gain of 127 - 2077. The gain is variable by using a 330 ohm resistor in series with a 10K ohm reverse log audio taper potentiometer as RG for IC2 (connected between PADS G1 & G2 of the schematic).

In the schematic R2 is shown as 2K2 as I had used this value for my HH2 R12. In my version of the HH2, I omitted R14 and I also socket mounted R11. When using the daughter board and a differential coil you must not use the HH2 R11 and R12, but remove these and use an appropriate damping resistor between HH2 L1 coil connection and daughter board X1-2. The differential preamp can be used with conventional HH2 coils by shorting X1-1 and X1-2.

This daughter board was designed in Eagle.

[KingJL]

As I understand layout have to be 2.5" x 1.5" measure on border around circuit?

The outer black border line is the dimension layer and it is 2.5" X 1.5".

[WM6]

I have a high resolution (encapsulated post script) pdf (1200dpi), but it way too large for posting in the forum (zipped it is 2M). If anyone wants it, the only option I have is email.

Thank you. Somewhat too late for me. I just hand-adapted PCB layout to my limited lab possibilities.
Maybe useful to someone others too? As I understand layout have to be 2.5" x 1.5" measured on border around circuit?

[KingJL]

Here you go. This is a bottom image (the board is designed as 1 sided).

I have a high resolution (encapsulated post script) pdf (1200dpi), but it way too large for posting in the forum (zipped it is 2M). If anyone wants it, the only option I have is email.

[KingJL]

I will create a pdf with the trace template. I will make sure that it is scaled acurately 1:1. May take me a couple of days to make sure it is right.

Here you go. This is a bottom image (the board is designed as 1 sided).

[KingJL]

Can you explain how you connect this front end to HH2 project. At which points in HH2 schematic?

I thought I would kludge together a little daughter boad with another INA217 configured as single-ended inverting with variable gain control (see attached jpg). Daughter-board would plug into IC6 socket.

Can you post black-white schematic of PCB too (without marked parts on it)?

I will create a pdf with the trace template. I will make sure that it is scaled acurately 1:1. May take me a couple of days to make sure it is right.

[WM6]

T

Initially, the backend is a HH2 that I have built with the reference connected to GND.

The TX section uses the elements of Carl's HH2 TX.

. The schematic and board image are contained in the attached zip file. The next step is to etch a PCB.

Hi KingJL

Interesting project.

Can you explain how you connect this front end to HH2 project. At which points in HH2 schematic?

Can you post black-white schematic of PCB too (without marked parts on it)?

[KingJL]

The zip file you posted has two copies of the same schematics contained inside (no board file).

Sorry about that! I'll post the jpeg of the schematic and the corrected board here: