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Archive for May 12th, 2017

AD8310 Log Amp Module: LF Response

The label atop a generic AD8310 Log Amp module seemed unambiguous:

AD8310 Log Amp module - overview

AD8310 Log Amp module – overview

Firing the HP 8591 tracking generator into the InHi SMA, terminating InLo (not shown above, for reasons you’ll see below), connecting the Out SMA to the scope’s Trace 1, and the spectrum analyzer’s sweep output to Trace 2 produced an oddity:

AD8310 Log Amp - 100 kHz 500 MHz

AD8310 Log Amp – 100 kHz 500 MHz

The upward-sloping ramp (lower trace) shows the HP 8591’s horizontal sweep, with the tracking generator tuning from 100 kHz to 500 MHz during the 20 ms sweep. The log amp output (upper trace) drops more-or-less linearly with increasing frequency, which seems odd. The tracking generator signal should be pretty much level and the log amp’s output should be more-or-less flat.

My oscilloscope tops out at 150 MHz. The displayed RF is down by 3 dB = 0.6 div at 1.5 division = 190 MHz into the sweep:

AD8310 Log Amp - 100 kHz 500 MHz - RF 50 ohm term

AD8310 Log Amp – 100 kHz 500 MHz – RF 50 ohm term

However, the RF looks pretty much flat up to 125 MHz and it’s still visible beyond 400 MHz, so I think the tracking generator is doing what it’s supposed to. If the RF were decreasing, then the trace would look different, methinks.

The response to a 60 kHz sine wave doesn’t look quite right:

AD8310 Log Amp - 60 kHz 1 Vpk

AD8310 Log Amp – 60 kHz 1 Vpk

Eyeballometrically, it might be a log response to the absolute value of the derivative: kinda flat on the ups-and-downs, kinda zero-ish at the tops-and-bottoms. Or maybe it’s the log response to a phase-shifted version of the input, with the lows corresponding to the zero crossings.

Documentation for the circuit seems nonexistent, because eBay. Fortunately, one can pop the top to reveal the straightforward PCB layout:

AD8310 Log Amp module - uncovered

AD8310 Log Amp module – uncovered

A closer look:

AD8310 Log Amp module - PCB detail

AD8310 Log Amp module – PCB detail

A capacitance meter says input capacitors C5 and C7 are both 10 nF.

A sketch of the circuitry:

AD8310 Log Amp module - input circuit

AD8310 Log Amp module – input circuit

The datasheet puts the terminating resistor on the other side of the input caps, where it surely belongs:

AD8310 Datasheet - Basic Connections diagram

AD8310 Datasheet – Basic Connections diagram

 

Achtung: the solder blob just to the left of C7 grounds the signal pin on the InLo SMA. Don’t connect anything to InLo which might take offense at having its output shorted to ground; the SMA terminator I used had no effect whatsoever.

The AD8310 chip (assuming that’s what it really is) has a differential input resistance = 1 kΩ and capacitance = 1.4 pF in parallel with R3, the 52.3 Ω terminating resistor, making the net resistance just under 50 Ω.

At 60 kHz, the input caps have a reactance of 270 Ω apiece, which means the “terminating” resistor is maybe 10% of the mostly capacitive input impedance seen at the InHi connector. That means the AD8310 inputs see maybe 10% of the input signal.

In fact, if you regard those three parts as an RC high pass filter and merge the caps into a single 5 nF unit, it rolls off at 620 kHz = 1/(2π · 52 · 5 pF). Obviously, it’ll be a fine differentiator at 1/10 the breakpoint frequency.

A simulation shows it in action (clicky for more dots):

AD8310 Log Amp module - input circuit simulation

AD8310 Log Amp module – input circuit simulation

The two 1 MΩ resistors provide a balanced DC path-t0-ground for R3 to keep the simulator happy.

The (+) input tends toward 0 dB as C5 tends toward a short, the (-) input tends toward ground as C7 does likewise, but their difference isn’t a constant value. Seeing as how a log amp should respond to small differences, methinks it’s hard at work.

The AD8310 data sheet says the scale factor is about 24 mV/dB between 10 MHz and 200 MHz, with no frequency dependence worth mentioning. Eyeballometrically, the output has a 240 mV = 10 dB straight-line decrease over the entire frequency range of that scope shot. It drops by 220 mV = 9.2 dB in the decade from 50 to 500 MHz, half of the 20 dB you’d expect from a first-order filter response.

The AD8310 has an internal 2 MHz high pass feedback loop to suppress low frequency input offset voltages. The doc recommends a 1 µF cap from OLFT to ground for frequencies down in the low audio range. One might solder the cap across the convenient pads labeled C8 below the chip.

Rearranging the input circuitry seems in order:

  • Move R3 outside C5 and C7, per the datasheet
  • Increase C5 and C7 to 1 µF -ish
  • Add 100nF – 1 µF bypass cap at C8

I have the uneasy feeling I’m overlooking something obvious …

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