New Bipolar Boost TX and Front End

[Mdtoday]

I have identified 2 areas of the TX-RX schematic/PCB board set that I would like to upgrade in verison 2,0 of the TX-RX board set:
(1)RX pre-amp... Get rid of the THAT1512 - OPA828 configuration and replace it with two OPA828's configured as a modified two op-amp instrumentation amplifier...............


(2) Add circuitry to assist the boost provided to the TX This assist pumps the boost by the voltage at "B" of the following schematic fragment. As configured, a B of about 9.6v provides fo an absolute flat 1.10A TX for 45 usec of a 50 usec TX pulse.


Also the plan is to modify the CMOD-A7I/F board to remove the unused/un-needed PMOD connectors and add circuitry to support a 20 or 24 bit ADC (LTC2378 or LTC 2389-24). Work on version 2.0 will not occur until late spring/early summer of 2020. That will give us time to test out the current configuration and determine what other changes might be needed.

I really like the proposed OPA828 configuration and boost circuit it makes good sense as does removing the unused IF PMOD connectors, it will make for a very nice compact design.
Yes, I agree, we need time to test the current version and take notes for improvements for sure.

I identified and purchased some raw display modules and other supporting components with view to design an all in one front panel PCB that includes the display, encoder and PMOD connectors to help improve the assembly and reduce cost.
The PMOD modules from Diligent are great but I thought we could improve the layout more to suit.
I have started the schematic and board size but have put it aside and will do more during my Xmas break.

[Mdtoday]

Agree... but sometimes (if you are lucky) you can pick up shorter lengths on ebay. Just found (and ordered) a 20' section ( @ $0.85/ft ) of Belden 9841, that may prove adequate for a mono. Capacitance C-C: 12.8 pF/ft, C-C-S: 23 pF/ft. The actual pulse seen by the inductor primary is 1.5 usec, but the test you performed does validate the primary secondary ratio.

Kind regards, J.L. King

That was a good pick up on the Belden 9841 and nice specs.

[KingJL]

Thanks, brings up the question. What is the minimum amplifier frequency cutoff? I've been trying for 1MHz with my testing, probably higher than needed. Think MPP calculates around 300kHz. If I calculate correctly, your amplifier with a gain of 300 would be around 150kHz. With a gain of 21, around 2MHz.

Actual measured with LtSpice with gain of 300 is 133 kHz (when the ac is set correctly).

[green]

Sorry, I missed the second part of your question... For a gain of 300 change R1 and R3 to 15k. In post #367, I incorrectly stated "The maximum gain si set by the ratio (R1/R4)+1"... it is actually set by the ratio R1/R4 with R1 = R3 and R2 = R4.

Thanks, brings up the question. What is the minimum amplifier frequency cutoff? I've been trying for 1MHz with my testing, probably higher than needed. Think MPP calculates around 300kHz. If I calculate correctly, your amplifier with a gain of 300 would be around 150kHz. With a gain of 21, around 2MHz.

[KingJL]

... For a gain of 300 would you change R1 resistance or add another stage of gain?

Sorry, I missed the second part of your question... For a gain of 300 change R1 and R3 to 15k. In post #367, I incorrectly stated "The maximum gain si set by the ratio (R1/R4)+1"... it is actually set by the ratio R1/R4 with R1 = R3 and R2 = R4.

[KingJL]

Thanks, does R1 and R3 need to be equal resistance?

Yes... for a 2 op-amp instrumentation configuration R1 and R3 are the same. Also R2 and R4 are the same. The maximum gain si set by the ratio (R1/R4)+1. For a true linear 2 op-amp instrumentation amplifier configuration, eliminate diodes D1,D2, D3, D4, D5, and C1. BTW: the reference common voltage does not have to be 1/2 of Vcc.
Kid regards, J.L. King

[green]

Thanks, does R1 and R3 need to be equal resistance? For a gain of 300 would you change R1 resistance or add another stage of gain?

[KingJL]

Been trying to follow your interesting thread. Wondering what targets you think might drive an amplifier with a gain of 21 into the nonlinear region.

It depends on what you consider a target, the inductance and configuration of the coil, the configuration of the amplifier, and the headroom of the amplifier circuit. I tend to view a target as anything the input stage might experience across the coil, originating from any source. In the circuit proposed, the headroom is +- 10v around the 10v common voltage. The zener diode controls the transition into the pseudo log response region and prevents the amplified signal from reaching the the headroom limit. With the specified zener, the nonlinear transition region starts about +- 3V, which would translate to a signal of ~143mv. The goal of this circuit is to prevent the pre-amp from going into saturation regardless of the input level thereby not having to deal with recovery whether it is a target, interference, or even the TX voltage during transmit. The gain of 21 shown in the attachment is for interfacing with a 20 bit ADC... to interface with a 16 bit ADC the gain might need to be 256x - 350x. To interface with a 12 bit ADC the gain might need to be >= 1000x. At 350x the non-linear transition would be realized with an input of about 9mv... at 1000x about 3mv

Kind regards, J.L. King

[green]

I have identified 2 areas of the TX-RX schematic/PCB board set that I would like to upgrade in verison 2,0 of the TX-RX board set:
(1)RX pre-amp... Get rid of the THAT1512 - OPA828 configuration and replace it with two OPA828's configured as a modified two op-amp instrumentation amplifier. Modified in the sense that the positive input section has a pseudo linlog response to eliminate any pre-amp saturation condition and provide infinite dynamic range of the input. The two OPA828 configuration also has better noise performance that the THAT1512 in low gain conditions (the THAT1512 gain in the current design has a gain of less than x6). The maximum gain is configured by the size of R1 in the following schematic fragment.
[ATTACH=CONFIG]48327[/ATTACH]

(2) Add circuitry to assist the boost provided to the TX This assist pumps the boost by the voltage at "B" of the following schematic fragment. As configured, a B of about 9.6v provides fo an absolute flat 1.10A TX for 45 usec of a 50 usec TX pulse.
[ATTACH=CONFIG]48326[/ATTACH]

Also the plan is to modify the CMOD-A7I/F board to remove the unused/un-needed PMOD connectors and add circuitry to support a 20 or 24 bit ADC (LTC2378 or LTC 2389-24). Work on version 2.0 will not occur until late spring/early summer of 2020. That will give us time to test out the current configuration and determine what other changes might be needed.

Been trying to follow your interesting thread. Wondering what targets you think might drive an amplifier with a gain of 21 into the nonlinear region.

[KingJL]

Proposed version 2.0 Upgrade to TX-RX schematic/PCB

I have identified 2 areas of the TX-RX schematic/PCB board set that I would like to upgrade in verison 2,0 of the TX-RX board set:
(1)RX pre-amp... Get rid of the THAT1512 - OPA828 configuration and replace it with two OPA828's configured as a modified two op-amp instrumentation amplifier. Modified in the sense that the positive input section has a pseudo linlog response to eliminate any pre-amp saturation condition and provide infinite dynamic range of the input. The two OPA828 configuration also has better noise performance that the THAT1512 in low gain conditions (the THAT1512 gain in the current design has a gain of less than x6). The maximum gain is configured by the size of R1 in the following schematic fragment.


(2) Add circuitry to assist the boost provided to the TX This assist pumps the boost by the voltage at "B" of the following schematic fragment. As configured, a B of about 9.6v provides fo an absolute flat 1.10A TX for 45 usec of a 50 usec TX pulse.


Also the plan is to modify the CMOD-A7I/F board to remove the unused/un-needed PMOD connectors and add circuitry to support a 20 or 24 bit ADC (LTC2378 or LTC 2389-24). Work on version 2.0 will not occur until late spring/early summer of 2020. That will give us time to test out the current configuration and determine what other changes might be needed.

[KingJL]

... Problem with a lot of the Belden cables is the MOQ...

Agree... but sometimes (if you are lucky) you can pick up shorter lengths on ebay. Just found (and ordered) a 20' section ( @ $0.85/ft ) of Belden 9841, that may prove adequate for a mono. Capacitance C-C: 12.8 pF/ft, C-C-S: 23 pF/ft.

On another matter, I re-wound some pulse transformers as per your specs of 14: 4 turns and did a quick check of inductance with average ratios of 140uH : 1.5mH, then set up a jig with mosfet to test voltages. With 5 volt 1.5kHz 20% duty cycle pulses driving the primary, there was 15 to 18v on the secondary, so the ratio is about right I think.

The actual pulse seen by the inductor primary is 1.5 usec, but the test you performed does validate the primary secondary ratio.

Kind regards, J.L. King

[Mdtoday]

I may have identified what may be a more suitable cable... at least it appears to be readily available... Belden 8729.

For mono coil cables, I would like to try Belden 1800F.

@KingJL, I have some samples coming from my supplier with a number of different cable types for testing.
Problem with a lot of the Belden cables is the MOQ but I'm working on that, we just need to identify the best fit.

On another matter, I re-wound some pulse transformers as per your specs of 14: 4 turns and did a quick check of inductance with average ratios of 140uH : 1.5mH, then set up a jig with mosfet to test voltages.
With 5 volt 1.5kHz 20% duty cycle pulses driving the primary, there was 15 to 18v on the secondary, so the ratio is about right I think.
I fixed the windings in place on the core with some Electrolube BLV
I will load them on the boards and test from there again before loading the Mosfets but looking good.

[KingJL]

This is good news, I have a selection of coils that can be used for testing, mono, concentric and DD.

I am going to try a mono and a DD without shielding. Since both RX and TX are full floating and the differential RX input is switched in polarity every TX cycle, I want to see if the capacitance ground effect will be cancelled in the filtering/integration process.

[Mdtoday]

I did some more checking... it appears that the 320uH TX will work... we have to tolerate a bit more TX current. 320uH/1 ohm @ 5V = ~1.23A coil current. 320uH/0.75 ohm @ 5V = ~1.36A coil current. 320uH/0.5 ohm @ 5V = ~1.52A coil current. But, the boost at those currents seems adequate.

This is good news, I have a selection of coils that can be used for testing, mono, concentric and DD.

[KingJL]

... You can try the ~320uH TX, but preliminary experimentation seems to indicate that < 500uH starts to lose to much boost. I am hoping to see the opposite effect of > 500uH with a good compromise of speed and exceptional boost being in the range of 750uH... we'll see...

I did some more checking... it appears that the 320uH TX will work... we have to tolerate a bit more TX current. 320uH/1 ohm @ 5V = ~1.23A coil current. 320uH/0.75 ohm @ 5V = ~1.36A coil current. 320uH/0.5 ohm @ 5V = ~1.52A coil current. But, the boost at those currents seems adequate.