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# The input and VCM of LTC6600-20

Hi all

When i use the amplifier of LTC6600-20, I have two questions

1、Does the input of the LTC6600 need AC-couple  or need DC- Couple with a common voltage when my input is differential.

2、 i connect the VCM of  ADC-AD9628 into the "VCM"  port of LTC6600, But the common voltage is 0.9V of AD9628 , it is lower than the spec of LTC6600 .

In datasheet, if i use 5V to power on the LTC6600, the VCM need larger than 1.5V（1.5V~3.0V）

So I want to check if the VCM is 0.9V, Whether it only affact the scale of the input signal or it will have some stability problems?

Thanks

1. Input Coupling: The inputs can be DC or AC coupled. Only thing is that the applied differential input CM voltage must be such that the device input pins (IN-, and IN+) do not dip below ground when operating from a single 5V supply, when using DC coupling. With AC coupling, you don't need to worry about that.

2. Output CM Voltage Range: The main issue with operating the output CM voltage at less than recommended values is the increase in distortion that you see in the attached plot from the datasheet: One way you can improve your case of output CM (pin 2) = 0.9V is run VMID (pin 7) at 0.9V as well (instead of leaving it floating to mid-supply). This way, you'd be operating at x-axis (pin 2 relative to pin 7) = 0V in the plot above.

I've put this in the LTspice example circuit below. You can experiment with this circuit for your exact set of conditions: Here is the simulation file for reference:

LTC6600-20 Example Single Supply EZ 2_28_22.asc

Regards,

Hooman

• Thanks

And i have read the datasheet, i find that the VMID is the V+/2( my V+ = 5V，V- = 0V)

So if i connect 0.9V to VMID, it means the sink current is nearly (5-0.9)/11k =  0.37mA

So how can check whether this will affect the ADC-AD9628. i do not find any specification of sink current in the datasheet of AD9628 Thanks. 