SDS800X HD Actual Use Cases

[mawyatt]

Since Bode is in dB, and Y axis is 20Log(V1/V2)

It is worth mentioning that Bode Plot doesn't have to be in dB. It can also use Vpp and Vrms.

Yes, but because of the expected range of the measurement (~1000X) better displayed in dB than in linear.

Best,

[mawyatt]

Setups for PSRR and Output Z measurements & plots.

Best,

[mawyatt]

Here's a KIA7810AP +10V regulator in TO-220 case (#10), we isolated the AWG (see setup above) with 2000uF and used 100 ohms for the sense resistor R in the output impedance setup. Load was established with Electronic Load (SDL1020X-E) at 100 ohms for a load current of 100ma.

So output impedance starts at 100Hz as Z ~ 100*(10^(-100.6/20)) or 933 micro-ohms and at 1MHz is Z ~ 100*(10^(-62.8/20)) or 72.4 milli-ohms.

In the next plot (#11) is the PSRR with a load current of 100ma. The range between 100 and 1KHz shows the PSRR improving, this isn't likely and caused by the setup coupling capacitance.

Edit: Added PSRR #13 which extends to 10MHz and shows some fixture setup resonates at ~3.5MHz.

Best,

[mawyatt]

Another interesting use of the SDS800X HD features, the Multi-Channel Bode Plot capability.

Here we have a 3rd Order Butterworth Active Low Pass Filter implemented with Equal Valued Components, see note #23 below.

https://www.eevblog.com/forum/beginners/calculation-of-the-3rd-order-rc-filter/msg5343509/#msg5343509

This circuit was implemented with 3 equal resistors and 3 equal capacitors and a single dual op-amp (LM358).

The Bode function allows one to "see" the voltage waveforms as they progress down the 3rd Order Filter Chain, from the 1st section C2 (note amplitude peaking which allows this overall transfer function to implement the 3rd Order Butterworth Response), followed by the 2nd section (C3) and the final result (C4) with the steeper roll-off response.

Edit: Added a High Pass by simply swapping the Rs and Cs in the Equal Valued 3rd Order Butterworth Active Filter.

Also, note how the op-amp artifacts (LM358) begin to effect the stop band LP performance and the HP upper frequency response. A better (faster) op-amp would yield better LP stop band and HP upper frequency responses.

The Rs were 1K 1% and the Cs were all 0.1uF 5%, which should yield a Low Pass and High Pass 3dB corner of 1.59KHz.

Best,

[mawyatt]

Here we've changed the R to 10K with same C (0.1uF), this should move the 3rd Order Corner down to ~159Hz. Here you easily see the 60dB/decade stop band response of the classic Butterworth Low and High Pass responses!

Edit: For those with some Complex Variable/Circuit Analysis interest, the Low Pass has a normalized transfer function of :

Vo/Vi = 1/[S^3 + 2S^2 + 2S + 1] and thus = root(2)/2 or -3dBV at S=j {w=1} at -135 degrees phase shift and at {w=0} Vo/Vi = 1 (0dBV)

For the High Pass:
Vo/Vi = S^3/[S^3 + 2S^2 + 2S + 1] and at S=j,  root(2)/2 or -3dBV at +135 degrees and at {w=0} Vo/Vi = 0

Check the posts below for R = 10K and C = 0.1uF, tau of 1ms {w = 1000, f = 159Hz} and -3dBV points, note phase shifts

Best,

[mawyatt]

Here's a couple plots showing how the op-amp dynamic output impedance limits the stop band rejection in active low-pass filters imploying the classic Sallen-Key (VCVS) configuration. The 2nd order VCVS type active low pass filter has a means for the input to couple thru to the output bypassing the op-amp by way of the feedback capacitor. The op-amp output impedance forms a voltage divider which limits the amount of input signal coupling thru to the output, thus lower output Z produces less coupling and better overall stop band rejection.

The above 3rd Order Low Pass Active Filter employs the mentioned Sallen-Key VCVS configuration to implement the 2nd order section of the 3rd order Low Pass Butterworth filter.

The first plot shows the Bode result, note the stop band limited attenuation in the overall filter C4 Green trace, the C3 Blue trace is the output of the VCVS section, and the C2 Magenta trace is the 1st op-amp output.

The second plot shows the same results except with a single added shunt resistor (1K) to the VCVS output op-amp section to the Vee supply rail. Note the dramatic improvement in overall stop band rejection (Green) and the VCVS (Blue).

What's happening is the shunt resistor draws static bias from the op-amp output npn transistor, forcing it into a more Class A rather than Class B bias condition, this significantly lowered the dynamic op-amp out Z and improving stop band rejection as shown. This simple technique is very effective with GP op-amps such as the LM358 type with the relatively weak Class B type output structure.

About 4  decades ago this simple concept was published in EDN.

Best,

[mawyatt]

Here are some LTspice simulations utilizing a generic LM358 model. The 1st plot shows the result above with 10K and 0.1uF components (compare with #28), the 2nd shows the added "shunt" 1K bias resistor (compare with #27).

Note how the simulations agree well with the actual circuit in the pass band, but begin to deviate as you move into the stop band.

This is a common occurrence with generic SPICE models, where the model doesn't represent the physical component well in certain areas, especially extreme cases.

The LM358 and most GP op-amps, have a very complex output impedance and rely on negative feedback to force the impedance to a low level. As the op-amp open loop gain deteriorates with increased frequency, the output impedance rises in an inductive fashion and can create issues with some circuits as shown here.

As most less seasoned tend to run to the simulations without totally understating the circuit under inspection, caution is advised, especially when "pushing" the limits for critical components.

In the case here the LM358 model is totally inadequate in the higher stop band region, and should be used with caution.

Anyway, more fun things for folks where you can compare physical and simulated circuits and begin the understand the limitations of both

Best,

[mawyatt]

The above begs the use of the Bode Function to plot the op-amp closed loop output impedance as shown in #23 for use with Linear Regulators.

Set up the op -amp with a bipolar supply and in unity gain configuration. Connect the + op-amp input to ground and "drive" the op-amp output with resistor R, connecting the signal source (AWG) to R.

From #23 then:

Z is R* 10^(Bode Display)/20

Here we have in 1st plot R at 10K with an LM358 with +-15V supplies, AWG at 10Vpp (1mapp). Note the inductive (rising with positive phase) output Z with frequency, starting at ~73 milli-ohms at 10Hz, rising to ~31 ohms at 10KHz and reaching a peak of ~1.24K ohms at 200KHz.

Changing R to 1K in 2nd plot for 10mapp "drive" signal yields ~82 milli-ohms at 10Hz, ~5 ohms at 10KHz and ~176 ohms at 200KHz.

Next plot is a simulation of an LM358 Spice Model, note the difference between the actual LM358 and model.

These effectively show how the output impedance of the LM358 is far from ideal, even at modest frequencies, and highly dependent on frequency and signal level in real life!!! Exactly why simulation models should be carefully evaluated and understood before committing to a design based upon such

Best,

[mawyatt]

Adding a shunt resistor to increase the output static DC bias current for the op-amp npn output transistor as mentioned above in post #30, where this reduces the LM358 output impedance which improves the Low Pass Filter Stop Band performance as shown, lets look at the LM358 output Z with and without the added DC bias.

1st is the output Z without the additional shunt R (1K) bias resistor, 2nd is with R from the output to Vee. Note the dramatic improvement in Z, from 5 ohms to 0.148 ohms @ 10KHz, a 30dB improvement

Maybe some folks at Siglent are watching and will realize the tremendous value of having Math functions available in the Bode Function, also would be nice to be able to superimpose previous plots (like reference plots), so one could show "before" and "after" comparisons 

Anyway, this little 12 bit DSO really packs some serious capability and can do much much more than just display a time domain waveform like the analog scopes of old 

Edit: If anyone is interested will be happy to explain what's going on with the various posts above wrt to the LPF and op-amp characteristics.

Best,

[mawyatt]

Here we have the SDS800X HD Bode Function showing the LM358 Op-amp Power Supply Rejection. Bode signal is "injected" by means of a coupling capacitor (1000uF) into the measurement supply rail (+-15V) using an "isolation" resistor (100 ohms). LM358 is configured as unity gain with output load of 10K, with scope CH1 connected to LM358 output and CH2 to injected voltage at supply rail. This configuration is swapped from normal Bode use so the display is in +dB rather than -dB to agree with TI Data Sheet as shown.

1st plot is of the + Supply Rejection, 2nd of - Supply Rejection and result from TI Data Sheet for LM358.

Note the fall off with frequency on both + and - Supply Rejection,  weaker - Supply Rejection common to most Op-amps, and the fair agreement with the TI Data Sheet Plot indicating this is likely a "real" LM358 and not counterfeit 

The Power Supply Rails are a common means for coupling noise and unwanted signals (SMPS) into op-amp based circuits, especially the higher frequency Supply Rail components due to the deteriorating nature of the Op-amp Supply Rejection with frequency.

So do you know for sure your LM358 is a real TI Op-amp or a counterfeit, this measurement is a useful means for verifying such

Best,