according to AD8336 datasheet, it is a single-ended input and output VGA, but my question is:
1. Is it possible to use its pre amplifier as a differential amplifier to changing a differential input signal to a singled-ended one pass to its VGA part? actually I want to use its pre-amp as a differential to single ended-part of a discrete instrumentation amplifier.
2. the AD8336 datasheet just offered two resistors for 12dB(x4) gain of biasing the pre amp, and said nothing about 26dB(x20) biasing resistor. which resistors are suitable for 26dB gain?
3. are there any app notes about nulling AD8336 offset voltage in high gain? or Do you have any solutions/circuits for doing it?
I highly appreciated to read your comments
1) In datasheet says that the preamp is a standard voltage feedback unity gain stable op-amp. Therefore, any standard configurations can be used here.
Memet said: actually I want to use its pre-amp as a differential to single ended-part of a discrete instrumentation amplifier
What do you mean?
TanQ for the reply
in datasheet says these about pre-amp configurations:
1) page 22. Circuit Configuration for Noninverting Gain: This amplifier is not designed for unity-gain operation. When using ±5 V power supplies, the suggested sum of the output resistor values is 400 Ω total......... Much of the low gain value device characterization was performed with resistor values of 301 Ω and 100 Ω, resulting in a preamplifier gain of 12 dB (4×).With supply voltages between ±5 V and ±12 V, the sum of the output resistance must be increased accordingly; a total resistance of 1 kΩ is recommended.
2) page 23. Circuit Configuration for Inverting Gain: The considerations regarding total resistance vs. distortion, noise, and power that were noted in the noninverting case also apply in the inverting case, except that the amplifier can be operated at unity inverting gain
so my question is:
1. for gain 26dB and supply voltage of ±5V and in noninverting configuration, are 931 and 49 ohm resistor ok or not?
2. If you look into AD8066 datasheet, you will find below figure(actually fig.60 in page 25.)
I want to know is it possible to make this circuit by a AD8066 as you can in the figure but replace AD8065 with AD8336's pre-amplifier or not?
if it is possible what resistors are suitable for biasing the pre-amp of AD8336? 500ohms are ok?
Memet said:I need a wideband, FET input, low noise, low distortion, double 5Volt supply, rail to rail output, low offset, unity gain stable op-amp!
Why are you sure about the need for RR output? Since it is a buffer this requirement will be justified if the DC component of the input signal is very close to the power rails. Are you all right with this?
I see in your diagram techniques typical of RF circuits, therefore, I assume low-resistance resistors here. Please confirm this and tell me what is the internal resistance of the source?
You named the maximum input amplitude 1 mV, then the output voltage of the ad8336 preamp will be 20 millivolts of amplitude. This is very far from the power rail and will not result in dynamic range limitation or clipping even in conjunction with the amplified offset voltage and the influence of bias currents. For example, you would need to get 4 volts of amplitude at the output, in which case it would be necessary to closely monitor the output offset to avoid clipping. In your case, you can ignore this and exclude this "reference voltage" from the design.
Do you agree with me?
you asked about why do I want to use the instr. amp. topology!
OK, I have to tell you more about this design. this device will be used in the power substation environment as sort of the electrical power devices analyzer. as you know in the power substation there are several type of interferences including the power frequency interference.
for reducing these interferences I use several solutions in my design such as proper PCB design, proper electronics component placement, proper casing and shielding design and etc.
also you have to know that the input source will be connected to the input via a long coaxial cable that passed though during of power substation environment. so reducing the effect of power frequency interference is one of the causes for using inst. amplifier topology in the input. Also the source has 50 Ohm output resistance.
the part of input circuit are:
1. protection circuits against over voltage and over energy that maybe occur in the substation.
2. Input AC/DC coupling switch
3. Input Active/Deactive switch
4. Pi Resistive attenuator with gain of attenuation equal to -54dB or -24dB with input/output impedance of 50Ohm. the input amplitude before attenuator is 1Vpp to 16Vpp and because of using AD8302 detector at the end of the circuit, nominally i have to limite the input to specific ranges by these resistors.
5. at the end, I used two didoes for protection of the op-amp pins according to the AD8066 datasheet information. these diode protect the opamp input against over voltage if the series resistor of the attenuator failed and shorted.
About TVS and GDT:
if the resistance between TVS and GDT is too smsll, such as low impedance pcb track, what you said is right. but if the resistance between these two are not low value and the 8/20us high energy pulse injected, these two component see the different voltage level at the same time. in my case these two will be connected with a 10cm, 50 ohm coaxial cable instead of a pcb track and they are not placed in a same board. Operation voltage of GDT is 75V and TVS is break down at 30V.
About output Rail to Rail feature:
yes, If you consider only the input range of 1uV to 1mV what you said is right. but I need this feature because of calibration scheme that I plane to use. one of signal I have to inject for calibration of the circuit is a 10Vpp sine wave waveform that will be injected to circuit after the attenuator with unity gain of all next amplifiers.
yes, If only consider the first AD8336 what you said about offset is right. but if you consider the second AD8336 with 60dB gain, the small mV offset pass through the first stage can saturate the output of the second amplifier. so the subject of nulling the AD8336 offset is important.
oK, right now Let's put aside the issue of TVS and GDT, also output Rail to Rail opamp, let me know your opinions about the rest of the circuit, such as how can I offset nulling the ad8336 circuit? how can I coupling the output of AD8336s to AD8302 via ADG1419? is the difference topology of the AD8336's pre-amp and its biasing ok or not? is using of AD8066 suitable for this purpose or not? AD8336 can be used with such large 22u to 47u capacitor as ac coupling capacitor in output ot between preamp and vga?
Thank you for your recommendations.
Yes, you are right that INA is immune to interference. The use of INA is justified in your case. But I'm looking at your scheme and I don't understand your specific implementation.
On the left is your idea. Using a coaxial transmission line in conjunction with an INA does not make sense. Grounding one of the inputs eliminates the advantage of INA-its insensitivity to common-mode interference.
On the right is a good idea that I understand. The twisted pair is enclosed in a grounded metal screen, which is the first stage of anti-interference. In addition, the interference induces common-mode voltages in both conductors of the twisted pair simultaneously, but the INA suppresses this, and only senses a useful signal between the two conductors.
INA has a symmetrical input, and this is its advantage. The coaxial cable is not symmetrical in principle. I don't understand how you want to do this and get high resistance to interference. I rely on the diagram you drew. Some of us are wrong...
I see on your diagram three points of decoupling on AC, therefore all cascades exist separately from each other on DC. Also, you mentioned the required bandwidth from 20 Hertz, so you can use as many capacitors as you want between stages and not worry about nulling.
I can offer several options. Since the input resistance of the attenuator is known and it is resistive, this resistance can serve as the lower side of the voltage divider. The upper side can be formed by a high-resistance resistor and a potentiometer.
You can also add integrators to these stages for automatic nulling and forget about this problem.
I hope you be as well as always and thank you so much for your replies.
ok, So I can add capacitors of any size between the amplifier stages without any worries.. that is a good news :))
you said: "Since the input resistance of the attenuator is known and it is resistive, this resistance can serve as the lower side of the voltage divider. The upper side can be formed by a high-resistance resistor and a potentiometer. " why did you offer this? you prefer this rather than resistive pi type attenuator?
at least, about automatic nulling by adding an integrator, would you pls draw a circuit based on AD8336 and an Integrator for automatic offset nulling of it? this is vey helpful for me to complete my idea. TanQ
Also, If I want to put a side INA topology for input, still AD8066 or single version of it AD8065 is suitable for first stage before AD8336? also do you have any ideas for reducing power frequency interference in the input? still I can keep that difference amplifier topology for pre-amp of AD8336. does it works?
Memet said:why did you offer this? you prefer this rather than resistive pi type attenuator?
No, this does not apply to the input part. This is just one of the possible ways to nulling the offset that I invented and suggested. It does not have any special advantages and looks sophisticated.
The integrator method can be implemented in this way
I want to warn you that this method, although very attractive, will give you a headache when implemented. You will need to use high-ohm resistors and large value capacitors to obtain a large time constant that will not affect the bandwidth. Do not rush to apply it. Stick to the decoupling capacitors first.
Memet said:how can I coupling the output of AD8336s to AD8302 via ADG1419?
Memet said:is the difference topology of the AD8336's pre-amp and its biasing ok or not?
What's bothering you here? Please explain what you mean by biasing?
Memet said:still I can keep that difference amplifier topology for pre-amp of AD8336. does it works?
If you can somehow connect the signal source to the amplifier with a symmetrical transmission line, this will be justified. More precisely, the use of INA here will be justified.
Memet said:still AD8066 or single version of it AD8065 is suitable for first stage before AD8336?
I do not know any special advantages of using a dual amplifier instead of two single ones, except for saving space on the PCB and the price.
Memet said: is using of AD8066 suitable for this purpose or not?
Since the input circuits are fairly low-impedance, a bipolar op-amp can give the main advantage here-a lower noise voltage. This is a feature of bipolar op-amps.
I highly appreciated for your replies.
would you pls recommend me some bipolar op-amp as AD8066 substitution with better noise, distortion and voltage offset spec also with sufficient GBW? TanQ
Carefully review these products, you may find something you like. They offer very low noise and these are not all possible offers, I'm just tired of viewing them all.
Be careful, I may miss something, you know about it:)
What do you think about LT1028?
Unfortunately, no, it's a bad idea. The data sheet says that it is stable with a gain of -1, which means, in other words, a noise gain of 2. It can't work as a buffer unless you take steps to increase the noise gain. But this doesn't make sense, since it will increase the noise at the amplifier output. Why do you need extra problems?
I recommend that you study MT-095, which describes how to connect a coaxial cable to an INA.
A few more questions: what is the maximum frequency of the calibration signal? And what do the resistors near the analog switch mean?