What this opamp is doing here - Page 1
Forum section: electronics / beginners

author: zulunation
author_rank: Contributor
timestamp: June 15, 2023, 05:17:51 pm
content: Hi everyone,A user A Helene to whom I am very thankful draw a DS1102E Rigol oscilloscope front-end.The image is herehttp://rigol.codenaschen.de/images/0/0c/DS1052E_HW58_PCB_Schematics_-_Ch1_analog_front-end.jpgI don't understand this schematics. I found in the discussion that AD8510 is used to split ac and dc parts of the input signal and add an offset to dc part. This I guess because ADC does not accept negative voltage.Am I right?If so how this technique is called. I know that opamp must have a feedback. Here the output is connected toinverting pin via capacitor marked as 10n.How this connection is called? I want to find more information.And next after AD8510 there are two diodes and next a 4 transistors. Looks like current mirror but I am not sure.What are they doing?Thanks,

author: jmelson
author_rank: Super Contributor
timestamp: June 15, 2023, 05:39:17 pm
content: No, I don't think it splits the AC and DC components of the signal. the 15 nF cap below relay 1B does the AC coupling as selected by relay 1B that shorts the cap out. The AD8510 ampifies the entire signal (AC plus DC) but it looks like most of the amplifying is done by the transistors to the right of the AD8510. The AD8370 is a variable gain amplifier, that also mixes in the DC offset and converts the signal to differential. I don't understand what the varicap is doing there, but I am guessing this forms a sample and hold with the AD8370.Hope this helps,Jon

author: MasterT
author_rank: Frequent Contributor
timestamp: June 15, 2023, 06:21:14 pm
content: Quote from: zulunation on June 15, 2023, 05:17:51 pmIt's one question, AD8510 is 8 MHz GBW according to data sheethttps://www.analog.com/media/en/technical-documentation/data-sheets/AD8510_8512_8513.pdfNot enough for 50 MHz o-scope. Circuits likely was design when high speed high GBW OPA were not available or expensive, so designer creates this "hybrid" or composite amplifier. Two path - high frequency and DC/ low frequency. First goes via n33 directly to 4 transistor G=1 buffer, 2 BJT below is a current mirror to improve JFET_BJT amp. Negative feedback "locked" all OPA+transistor composite amplifier, doing so ad8510 corrects JFET offset precisely.

author: dietert1
author_rank: Super Contributor
timestamp: June 15, 2023, 07:49:05 pm
content: I think the opamp is a low frequency servo to reduce/remove the temperature drift of the discrete buffer. It uses negative feedback from the output of the buffer to control DC into the buffer. The 4M7 resistor and the n33 capacitor add the servo signal and the input RF for the buffer input.Regards, Dieter

author: zulunation
author_rank: Contributor
timestamp: June 17, 2023, 07:01:49 pm
content: >>servo signalDieterCan you explain what is servo signal?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: June 17, 2023, 07:22:19 pm
content: I am with MasterT on this. The OP amp is really mostly a DC amp. The negative feedback to the amp is thru a cap so it is just the analog signal feeding back. Maybe even cancelling most of the analog thru the OP amp out.

author: magic
author_rank: Super Contributor
timestamp: June 17, 2023, 10:33:22 pm
content: It's a somewhat complex circuit which is an almost-unity gain amplifier with 100MHz or more bandwidth and preservation of accurate DC level. (But the offset control thing can be used to shift the signal up or down by a controlled amount, as set by a knob on the scope). DC accuracy is the reason for including the opamp chip because discrete transistor amplifiers tend to suck at it, particularly if the circuit is subjected to high temperature variations and this one is - those scope PCBs get quite hot after prolonged operation.The diodes constrain the signal at that point to be between -5V and 5V, nothing more to them. The transistors are not a current mirror; the lower ones are constant current sinks providing bias for the upper ones, which are a JFET source follower and a BJT emitter follower. These two transistors make a voltage follower buffer with high input impedance and low output impedance, voltage gain of almost unity and some unpredictable offset voltage equal to Vgs of the JFET plus Vbe of the BJT. The JFET is drawn upside down, by the way, the source should be down.At high frequencies, the input signal is coupled through 0.33nF capacitor straight to the JFET and overrides the 4.7MΩ signal coming from the opamp. Such frequencies reach the output through the transistor buffer at almost unity gain and completely bypass the opamp.At low frequencies, capacitors can be regarded as open circuits and ignored. Input signal is first divided ~5x and reaches the noninverting input of the opamp. The opamp drives the JFET/BJT buffer and takes feedback from its output. Feedback is divided ~4.7x before reaching the inverting input, giving 4.7x noninverting gain at the final output (TP2.1). Overall gain including initial division is almost unity and it ought to be matched with the gain of the high frequency path.There is a transition band of medium frequencies, where feedback from (10k+2k)/3k couples through 10nF and overrides the main feedback. In this range of frequencies, the opamp works as a 5x noninverting amplifier and drives the JFET with this signal. The result is that the opamp's output follows the input signal exactly and nothing interesting happens - both the 0.33nF and 4.7MΩ apply the same signal to the JFET. This "glues" the two different modes of operation together and provides a smooth transition between the low frequency and high frequency paths.Careful reader will note that gain of this circuit is determined differently for different frequencies and hence may not be exactly equal. I recall simulating this circuit a few years ago and finding that gain below a few kHz was predicted to be different than above a few kHz. This was likely because my sim models didn't match the reality of transistors used in those Chinese scopes (Rigol, Hantek - they both seem to use such frontends). One may hope that they tweaked those various small resistor in series with the larger feedback resistors to minimize gain variations in real world and that production is somewhat repeatable.Another interesting quirk is that at high frequencies, the opamp runs out of gain and its output pin no loner tracks the input signal. Then, the 4.7MΩ resistor appears to be grounded and acts as additional loading on the input, AC coupled to it by the 330pF. It appears as additional resistance in parallel with the normal 1MΩ input load (consisting of 806k + 392k/2), so input resistance decreases to ~0.8MΩ. I'm lazy and haven't done the math, but it's possible that this only happens at frequencies where input impedance due to the 20pF input capacitance is already much lower than 0.8MΩ anyway.update: 20pF capacitor impedance is down to 0.8MΩ starting near 10kHz and any half-decent opamp can maintain such bandwidth at closed loop gain of 5x easily.