Products > Test Equipment
Hantek 2000 series - 2C42/2C72/2D42/2D72
gf:
I'm adding photos of the two major AWG "regions" on the board (including the marked resistor).
69B = 5.11k
gf
JimBeam:
Ahh, that's perfect!! THANK YOU VERY MUCH, gf :-+
That means R65 is 5,11k (EIA-96 code 69B).
Edit: I overlooked, that you already wrote the value, so my comment was superflous. I should read post from the beginning to the end - but I was attracted by the pictures before reading...
gf:
--- Quote from: JimBeam on March 05, 2019, 06:37:35 pm ---I just did some calculations around the DAC:
With an Rset of 2,2k we get a fullscale output current of around 18mA, which results in a voltage drop of about 0,9V across the 50 Ohm output load (This disregards the input impedance of the OpAmp input dividers, but as I used 51 Ohm resistors anyway, the difference should be neglectible.)
As the gain of the output driver is 3,733 (560/150 Ohm) the fullscale output voltage at the BNC (without load) is 0,9V * 3,733 = +/- 3,36V.
So the DAC range isn't fully used - or is there a mistake in my calculation?
--- End quote ---
a) The voltage gain is lower than 560/150, since you neglected the 50 Ohm resistor.
b) On the other hand, the voltage gain applies to the voltage difference between the DAC outputs, which is higher than the single-ended AC voltage between one ADC output and GND.
c) Just applying a factor of 2 - in order to account for (b) - would not be valid either, since the AC voltage is not the same at both ADC outputs, because the current sources drive into different impedances. The inverted output of the DAC (leading to IN+ of the amp) is terminated with (50 Ohm || (150+560 Ohm)) == 46.7 Ohm towards GND, but the effective termination impedance of the non-inverted DAC output towards GND is affected by the feedback from the amp output, and is lower.
EDIT:
A more detailed analysis/calculation of this kind of ADC termination circuit can be actually found e.g. here: http://www.ti.com/lit/an/sbaa135/sbaa135.pdf. They also don't consider voltage gain, but rather transimpedance of the amp, as the inputs are current-driven. Quote from this paper:
--- Quote ---...This relatively simple-looking circuit is, however, not giving a matched gain nor a matched input impedance for the two current source outputs...
--- End quote ---
Anyway, I share the conclusion that the DAC not fully utilized. But it may make sense to leave a couple of percent headroom for gain/offset adjustments in the digital domain.
(1) DC bias measurement:
On my board, Rset = 25B = 1.78k (not 2.2k). If the IC were a DAC902, this would correspond to IOUTFS = 22mA. However, it is not a genuine DAC902, but a chinese chip with unreadable markings and unknown specs. So we can only guess. At least it seems to be plausible that the specs are similar.
When I select the "Arb4" signal (-> eventually DC with 50% digital count of full-scale), then I measure about 500mV DC each at the inverted and non-inverted DAC output pins, and the EL5166 output is about 0V (all readings against GND). At this bias point, each DAC output is loaded with 50 Ohm || (150+560 Ohm) == 46.7 Ohm towards GND. 500mV / 46.7 Ohm = 10.7mA at each output, and this is indeed pretty close to the predicted IOUTFS / 2. Looks reasonable :-+
(2) AC measuremt:
For simplicity it is IMO sufficient to consider only the AC current at the inverted DAC output whose termination impedance is not affected by the amp feedback (-> the AC current at the other DAC output is supposed to be the same, just inverted). With the AWG set to a square wave with an amplitude of 2.5V, I measure 870mVpp at this ADC ouput. 870mVpp / 46.7 Ohm = 18.6 mApp. And yes, that's only about 85% of IOUTFS.
[ My 6074BD scope is not so accurate, though, so please grant sufficient tolerance to my measurements. ]
gf
gf:
Follow-up to my previous message:
I calculated the gain accodring to the TI paper (neglecting beta), based on the nomimal resistor values on the board, and I get a total transimpedance of 280 Ohm (or in other words 280 mV/mA), where 280 = 50*560/(50+150)*2. I.e. 20mApp DAC current corresponds to 5.6Vpp at the opamp output (if there weren'n the clipping, of course).
I also tried to determine the DAC's IOUTFS a bit more exactly. The voltage at Rref measures 1.11V, which is obviously lower than DAC902's internal reference voltage of 1.24V. There is no voltage at the REFin pin (DAC902 were supposed to ouput the buffered internal reference voltage here, if I understand the block diagram correctly). I can't measure the 1.78k Rref in-circuit - some of the current seems to bypass the resistor via a different path - so I take the marked value as granted. This leads to a predicted IOUTFS of 1.11/1780*32, i.e. almost exactly 20mA.
The termination of the DAC's Iout- to ground measures 47.1 Ohm (nominally calculated was 46.7).
And With "Arb4" waveform (-> 0V DC signal), I read a voltage of 467mV at Iout-. That's an Iout- of about 9.92mA.
Iout+ splits up into 9.4mA to ground (through 50.1 Ohm) and 0.67 mA through a 150 Ohm resistor to the opamp's IN- pin.
So Iout+ is about 10.1mA, and the sum of the DAC's measured Iout- and Iout+ currents is almost exactly 20mA as well.
The nubers fit reasonably with each other.
If the reference voltage of your genuine DAC902 is actually 1.24V, as specifiled in the datasheet, then you likely need a slightly larger Rref (1980 Ohm), in order to get an IOUTFS of 20mA as well.
I'm also not sure, whether some calibration data (gain/offset correction) are stored in the device's flash. If they do exist, then they are certainly missing or incorrect in an "upgraded" 2Cx2. The notceably different I+ and I- currents measured with "Arb4" signal suggest that a digital offset correcetion may have been applied, since the measured voltage at the opamp output is indeed as low as 2.5mV, when it is supposed to be zero.
EDIT: I'll try to measure the digital count at the ADC data lines as well, which lead to the observed output, but I guess it's not 2048.
gf
gf:
@JimBeam:
I have now also measured the digital values sent to the DAC for 0V, 2.5V and -2.5.
Set voltageDigital DAC input valueMeasured voltage at EL5166 output0V20622.5mV2.5V2382.49V-2.5V3886-2.50V
=> I.e. from the range 0...4095, the sub-range 238...3886 (5.8%...94.9%) is utilized on my device in order to generate -2.5...+2.5V output voltage.
The digital values are so weird that I'm now almost sure that they are different on each device, depending on its individual calibration.
Btw, obviously lower digital values represent higher output voltage, because IN+ of the amp is fed from the DACs IOUT-.
And 2062 also happens to be (3886+238)/2 (as expected) :).
EDIT:
Btw, I'm surprised that there was no clipping at -2.5V. But this was measured w/o load. With 50 Ohm at the AWG output, the voltage drops to -2.39V. Also measured my EL5166 supply rails again => -3.41V and 4.92V.
gf
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version