VCA Multiplying DAC.
Posted: Fri Sep 20, 2019 7:41 am
I am on a VCA design where I need high headroom, high accuracy ( resolution and stability )
I am thinking to use 16 bits MDACs.
DAC8812, AD5545. From TI, AD they look the same.
The data sheets give -104dB THD at 5Vpp. That is 0.0005%. How do they perform at higher levels ?
The AD5545 only mentions AC ability with Vref +12V / -12 V, the DAC8812 ??
Who knows, how close to max rating do they have a low thd ?
These MDACs are monotonic and I do need monotonicity.
The data sheets warns that the I-V op amp should be a precision one, but does not help much.
It says low ibias is needed to insure monotonicity, and mentions low Vos.
I understand about ibiais, because of the impedance seen from the op amp input that varies with the 16 bits code. However I do not see
about Vos.
IMO, Vos has no impact on monotonicity, I would like to make sure, though.
OPA 1612 seems, not good enough about ibias.
So I am thinking to use OPA 1642 for it's FET input: 20pA max ibias, ios, despite it's 3.5mV max Vos. However I wonder about Vos temperature drift that is not specified.
A higher cost alternative is OPA828 a FET input with very low Vos and drift.
I am thinking to use 16 bits MDACs.
DAC8812, AD5545. From TI, AD they look the same.
The data sheets give -104dB THD at 5Vpp. That is 0.0005%. How do they perform at higher levels ?
The AD5545 only mentions AC ability with Vref +12V / -12 V, the DAC8812 ??
Who knows, how close to max rating do they have a low thd ?
These MDACs are monotonic and I do need monotonicity.
The data sheets warns that the I-V op amp should be a precision one, but does not help much.
It says low ibias is needed to insure monotonicity, and mentions low Vos.
I understand about ibiais, because of the impedance seen from the op amp input that varies with the 16 bits code. However I do not see
about Vos.
IMO, Vos has no impact on monotonicity, I would like to make sure, though.
OPA 1612 seems, not good enough about ibias.
So I am thinking to use OPA 1642 for it's FET input: 20pA max ibias, ios, despite it's 3.5mV max Vos. However I wonder about Vos temperature drift that is not specified.
A higher cost alternative is OPA828 a FET input with very low Vos and drift.