Re: isolated DAC. PWM ( 20khz ) to analog 0-10Volt
What you want is when there is no input, the output is zero.
The 1st amp is an inverting amp. so then there is no input, the - input is pulled high, making the output low.
I would increase R4 to give the inverting buffer (1st amp) a very high gain. Make R4 1Meg and see the result.
R6 and R7 are ok at 33k or 100k since they are just a voltage divider.
Larry
Manufacturer of CNC routers and Viper Servo Drives
www.LarkenCNC.com and www.Viperservo.com