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I/V stage for ES9028Q2M
I/V stage for ES9028Q2M
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Old 7th February 2018, 04:53 AM   #1
gandolf is offline gandolf  United States
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Question I/V stage for ES9028Q2M

Hi,
I am trying to design an I/V stage for ES9028Q2M DAC. I was first considering the Zen style, using a common gate amplifier to present low impedance to the DAC. But soon I have encountered problems:
Click the image to open in full size.

First, in order to really get low input impedance, the transistor needs to be biased to saturation region, this requires a certain amount of Vds (> Vgs-Vth), assume 4V. In addition, the bias current should be much larger than the DAC output current to make the DAC output a 'small signal' (keep the bias point stable). The ES9028Q2M can put 7mA out, the bias current of the common gate should be around 70mA. With 150R resistor, the voltage drop is 10.5V. The third one is the DAC output is NOT 0V, it is AVCC/2, 1.65V in my case. Thus the input common mode of the common gate should match this voltage. A resistor is not possible since the resistance will be limited and this will draw some of the DAC output currents to ground. An LDO may be the only solution. This further push the supply of the amplifier to as high as 16V. I would rather NOT to have such a high voltage rail around a mainly 3.3V circuit, let alone the required 2X LDOs for each channel.

Here is my question, why don't people use low noise high bandwidth fully differential amplifier from ADI or TI to implement the I/V stage? Like following:

Click the image to open in full size.
For example, AD8138 from ADI is a nice amplifier. It has a 300MHz+ bandwidth, 1000+ V/us slew rate, very low noise, very low distortion, works with 3.3V single rail and adjustable input/output common mode. What is the drawback to using such an amplifier to implement the I/V stage?
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Old 7th February 2018, 02:33 PM   #2
abraxalito is offline abraxalito  United Kingdom
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From a quick look at the datasheet, the 'very low noise' description only applies above the audio band. There does look to be rather a high noise corner frequency. In comparison to a discrete I/V stage, a disadvantage for an IC would be its classAB output stage, leading to noisy power supplies. Incidentally, to avoid the need for higher voltage rails you can adopt current source biassing for the common-gate transistor. This would allow only the signal current to pass through the I/V resistor rather than the whole of your 70mA.
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Old 7th February 2018, 03:13 PM   #3
Keruskerfuerst is offline Keruskerfuerst  Germany
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Can you give the link to the datasheet of that DAC chip?

The common I/V stage is a high speed OP.
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Old 7th February 2018, 10:59 PM   #4
gandolf is offline gandolf  United States
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Quote:
Originally Posted by abraxalito View Post
From a quick look at the datasheet, the 'very low noise' description only applies above the audio band. There does look to be rather a high noise corner frequency. In comparison to a discrete I/V stage, a disadvantage for an IC would be its classAB output stage, leading to noisy power supplies. Incidentally, to avoid the need for higher voltage rails you can adopt current source biassing for the common-gate transistor. This would allow only the signal current to pass through the I/V resistor rather than the whole of your 70mA.
I still need an opamp as I want to keep the supply to 3.3V across the board. The entire supply would now enough to provide enough Vds for most MOSFET. After reviewing more opamps, I would like to use LT1819 instead the ADI one. It has 400MHz bandwidth, 2500V/us slew rate. Noise wise, 6nV/sqrt(Hz) after corner frequency and ~40nV/sqrt(Hz) at 100Hz, should be able to claim low noise this time.

Tentative schematic:
Click the image to open in full size.

Transient simulation with 7mAp-p current input @1KHz:
Click the image to open in full size.

AC simulation (Unity gain stable!!):
Click the image to open in full size.
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Old 7th February 2018, 11:00 PM   #5
gandolf is offline gandolf  United States
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Quote:
Originally Posted by Keruskerfuerst View Post
Can you give the link to the datasheet of that DAC chip?

The common I/V stage is a high speed OP.
I am sorry the datasheet is protected by NDA (ESS.....).

Yeap, I am trying to use an OP to implement my I/V
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Old 8th February 2018, 12:59 AM   #6
quanghao is offline quanghao  Viet Nam
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Hi gandolf!

Vcm you want = 0V?

Follow data off ESS Vcm i see the sample image

thanks
Attached Images
File Type: png diffopamp.png (112.4 KB, 66 views)
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Old 8th February 2018, 02:11 AM   #7
gandolf is offline gandolf  United States
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Quote:
Originally Posted by quanghao View Post
Hi gandolf!

Vcm you want = 0V?

Follow data off ESS Vcm i see the sample image

thanks
No, Vcm should not be 0V.

Yes, in the actual circuit there will be several decaps at the Vcm node.
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Old 8th February 2018, 02:26 AM   #8
quanghao is offline quanghao  Viet Nam
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Quote:
Originally Posted by gandolf View Post
No, Vcm should not be 0V.

Yes, in the actual circuit there will be several decaps at the Vcm node.
please go up 150R to 2.2K to 10K and one cap to GND. i think 150R is low value!

thank
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Old 8th February 2018, 02:36 AM   #9
gandolf is offline gandolf  United States
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Quote:
Originally Posted by quanghao View Post
please go up 150R to 2.2K to 10K and one cap to GND. i think 150R is low value!

thank
Thanks for the suggestion.
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Old 8th February 2018, 04:01 AM   #10
DPH is offline DPH  United States
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Gandolf, go look for something like the 9018 or 9016 Datasheet online, the spec design is essentially a LME49720 configured as I/V's, pretty much classic design.

Also, the parts you're suggesting aren't low noise at audio bandwidth, maybe something like the OPA1632, but feel free to simply use a dual opamp instead.

It's also not difficult to make a center-tap reference from a precision resistor divider and a capacitor (for stability). If you look for JensH's posts in one of the 9038q2m threads, he's got a circuit. Scott Wurcer threw in a nice general form (and some things to think about) for an I/V in the AD797 http://www.analog.com/media/en/techn...eets/AD797.pdf pg 17.

There's a bevy of ways to do this. A quick/but not ridiculously quick low-noise general purpose opamp would be excellent. If you want to go nuts, look at an OPA1612 or better ADA4898. Mind your layout though.
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