The 100MHz Probe:

When I decided to add the 10MHz and 1MHz versions I had to tweak the 100MHz probe a bit. All of the versions use the same PCB layout. The higher supply voltage on the slower versions forced me to change the larger tank bypass capacitors from 0402 size to 0603 because it is difficult to get large 0402 ceramic caps rated above 16V. Also, the input resistor string was changed from 3x3.3Meg + 62k to 4x2.49Meg using 0805 resistors (see reason below).

The 10MHz Probe:

Technically, not a redesign but constrained by keeping the same PCB layout as the 100MHz probe. It was difficult to find suitable opamps. They had to meet the following criteria:

1. A dual opamp in a MSOP8 (or VSSOP8) with the same pinout as the LTC6269-10, and a single opamp in a SOT26 or SOT25 with the same pinout as the LTC6268.
2. Supply voltage > 12V (More supply voltage yields a higher differential input signal)
3. Gain-BW product > 20MHz - 60MHz, depending upon how gain was allocated to the first and second stages.
4. Slew Rate > 400V/µs. (6Vpeak x 10MHz x 2π)
5. pA input bias current (for low noise)
6. Rail-to-rail output swing would be nice.
7. Less expensive than the LTC6268/9 (this is not difficult).

Needless to say this narrowed the field a bit. After loosening a few of the criteria I finally settled on the OPA2810 and OPA810, which has the following specs:

1. Available in a VSSOP and SOT25 package.
2. Max supply voltage 24V
3. GBW = 70MHz
4. Slew Rate = 192V/µs
5. 2pA input bias current typical
6. Rail-to-rail in/out.
7. Less than $5.00 for the OPA2810, and around $3.50 for the OPA810.

I settled on a 15V supply voltage, which would yield an output swing of 14Vpp with the rail-rail output. Since the gain is set at 1/10 V/V the differential input voltage swing is set at 140Vpp. The higher supply voltage also means that less input attenuation would be required and still meet the common mode range needed, so I decreased the attenuation from 1/250 V/V to 1/100V/V. 

I also thought, mistakenly, that reducing the resistance of the attenuator from 10MegΩ to 5MegΩ would reduce the Johnson thermal noise. This increased the input capacitors from 10pF to 20pF to keep the same crossover frequency.

With a GBW of 70MHz the OPA2810 won't have a -3dB BW with a gain above 5 or 6. I set the gain at 5, which makes the second stage gain an easy 2. You want to have the first gain stage as high as possible to suppress input referred noise from the later stages. You also want to have the last stage set the bandwidth of the probe. The max CM voltage x attenuation + max single-ended input voltage x attenuation x first stage gain must be less than 7.25V:  353V/100 + 70V/100 * 3 = 5.63V. The gain of 3 (instead of 5) is because the input is single-ended and has a common mode component to it. If the input signal was differential the max output swing drops to 5.28V.

I had to give up having full bandwidth with a 70Vpeak input swing. The slew rate of the last stage will only allow a 3Vpeak sinusoid without hitting the slew rate limit.

The datasheet suggests keeping the feedback resistors below 2k to avoid degrading the noise performance of the opamp. The max power dissipation of an 0603 resistor is usually less than 0.1W. A quick check on the feedback resistor max dissipation in the second stage = 10.9mW, no problem there. A fat finger check of the total power dissipation of the probe -- 4mA x 3ma/opamp + 3mA (for LED + rail splitter) is less than 250mW. It should be pretty cool.

The bypass caps were changed in accordance with those suggested by the datasheet. This forced a change to the large 2.2μF and 4.7μF capacitors -- changing the footprint to 0603 to make it easier to get them rated for 25V or 35V. (And this in turn caused the update to the 100MHz PCB layout.)

uh..oh...

At this point I found an error in the design of the old diff probe. When I was searching for parts on Digikey and JLCPCB I noticed that all of the 0603 resistors had a max working voltage of 75V. The old design had three 3.3Meg resistors in series with a 62k, so almost all of the input voltage was developed across those three resistors, which exceeded the max working voltage rating. To fix the problem, all of the resistors in the input string must be equal -- in this case 1.24Meg, and their sizes increased to 0805, which increases the max input voltage to 4x150V = 600V. The input voltage requirement is for 360Vpeak + 70Vpeak = 430V. When I changed the resistors to 0805 on the layout I overlapped the courtyards to keep the overall dimensions of the PCB the same.

The 1MHz Probe:

I thought it would be easy to find cheap opamps with GBW > 10MHz and slew rates > 45V/µs, but I gave up when I couldn't find one with the correct package that would take a 15V supply. So the 1MHz probe uses the same opamps as the 10MHz probe and this makes the design pretty simple.

The attenuator is unchanged at 1/100 V/V. Since opamp GBW is a non-issue the first stage gain is raised to 8 V/V, and the second stage lowered to 1.25 V/V, for better noise performance. A quick check to make sure that the output voltage swing of the first stage is still within the supply rails: 353V/100 + 70V/100*4.5 = 6.7V, for a single-ended input.

The bandwidth of the first stage is a don't care -- a 10pF feedback capacitor sets it at 10MHz, but in reality the GBW of the opamp will limit the bandwidth to less than 70MHz/8 = 8.75MHz. The probe bandwidth is then set by the second stage to about 1.5MHz.

Power dissipation is about the same and the bypass caps are the same.

Powering the 10MHz and 1MHz probes:

I suggest to use a USB-C trigger board (set to output 20V) coupled with the VREG daughter board that uses the TL432 shunt regulator design. With this arrangement, the daughter board will only dissipate about 75mW and provide a tightly controlled 15V to the probe. The overvoltage protection and the second pass transistor can be deleted, for a pretty cheap solution. Details about this are noted in the schematic.

Status:

[2025-07-12] The revised probe PCB (V3) and the updated daughter board PCBs are being fabbed at OSH Park. My order of parts to build the 1/10 MHz probe have been shipped.