G'day all,
I now have a pair of 3478a to compliment a 3457a. I knew one of the 3478a was off compared to the other two meters, so I knocked up a bit of a hack with a homebrew GPIB controller to present some data to watch.
As you would assume, with a 3457a in 100NPLC, I get 10 readings of the 3478a(s) at 10NPLC for every 3457a reading, so I store all 10 readings from both 5.5 digit meters and then average them.
- The first 3 values are the 3 meters (3478a 3478a 3457a) last raw reading. The reading from the 3457a uses the HIRES register also.
- The next two are the averaged readings from the 3478a(s) over the last 3457a reading period (I just re-trigger them after each read until the 3457a asserts SRQ).
I then just do a simple subtraction between both 3478a and the 3457a and break that out in voltage and PPM of the actual reading (so Meter 3 - Meter 1 and then Meter 3 - Meter 2). As usual, there's way more digits there than there is accuracy, but it's interesting to watch the readings track with different voltages.
27.8513 : 27.8498 : 27.84987 : --- 27.8513 : 27.84976 --- -1.429 mV : -51.3 ppm : 108.7 uV : 3.904 ppm : 10,0
27.8513 : 27.8497 : 27.84984 : --- 27.85129 : 27.84973 --- -1.455 mV : -52.26 ppm : 104.9 uV : 3.767 ppm : 11,0
27.8512 : 27.8497 : 27.8498 : --- 27.85124 : 27.8497 --- -1.436 mV : -51.57 ppm : 104.9 uV : 3.767 ppm : 12,0
27.8513 : 27.8497 : 27.84978 : --- 27.85128 : 27.84969 --- -1.507 mV : -54.1 ppm : 85.83 uV : 3.082 ppm : 13,0
27.8513 : 27.8497 : 27.84979 : --- 27.85127 : 27.8497 --- -1.482 mV : -53.21 ppm : 89.65 uV : 3.219 ppm : 14,0
27.8513 : 27.8497 : 27.84984 : --- 27.8513 : 27.84974 --- -1.457 mV : -52.32 ppm : 101.1 uV : 3.63 ppm : 15,0
27.8513 : 27.8497 : 27.84984 : --- 27.8513 : 27.84971 --- -1.463 mV : -52.53 ppm : 125.9 uV : 4.52 ppm : 16,0
27.8513 : 27.8497 : 27.84983 : --- 27.8513 : 27.8497 --- -1.467 mV : -52.67 ppm : 135.4 uV : 4.863 ppm : 17,0
27.8512 : 27.8497 : 27.8498 : --- 27.85127 : 27.84969 --- -1.474 mV : -52.94 ppm : 104.9 uV : 3.767 ppm : 18,0
27.8513 : 27.8497 : 27.8498 : --- 27.85127 : 27.8497 --- -1.474 mV : -52.94 ppm : 97.27 uV : 3.493 ppm : 19,0
27.8513 : 27.8497 : 27.84982 : --- 27.8513 : 27.84971 --- -1.482 mV : -53.21 ppm : 106.8 uV : 3.835 ppm : 20,0
27.8513 : 27.8497 : 27.84982 : --- 27.8513 : 27.84973 --- -1.480 mV : -53.15 ppm : 87.74 uV : 3.15 ppm : 21,0
27.8513 : 27.8498 : 27.84982 : --- 27.8513 : 27.84972 --- -1.476 mV : -53.01 ppm : 103.0 uV : 3.698 ppm : 22,0
27.8513 : 27.8497 : 27.84984 : --- 27.8513 : 27.8497 --- -1.457 mV : -52.32 ppm : 145.0 uV : 5.205 ppm : 23,0
All the math is 32 bit floating point.
Now I might be able to use the information in the 3478a Cal RAM thread to tweak the cal values live and watch what they do relative to both the other 3478a and the 3457a.
I'd forgotten how much you can actually get done with GPIB when you get into it.