P
Piotr Wyderski
Guest
Hi,
TL;DR: a precision linear MOSFET pre-driver capable of putting negative
voltage on the gate in order to shut the current flow down. Quickly and
hard to eliminate spurious noise-induced turn-ons.
Detailed:
I have a precision unbuffered 16-bit DAC that outputs 0--4.095V. The
analog subsystem uses +12/-5V supplies in order to place the opamps
(OPA4192)
well within the linear region. Now I would like to buffer the VDAC
voltage, but also augment the buffer with the ability to output some
negative VNEG voltage in shutdown state. The closer VNEG is to the VEE
rail the better, but the exact value is unimportant. I can see 3
possible scenarios based on a DG419 SPDT switch:
1. Create a follower and add the mux to the input, switching between
VDAC and VEE. But this can add noise/nonlinearities to the precision path.
2. As above, but put the mux at the follower's output. This would add me
~30 Ohms to the output impedance.
3. Connect the DAC directly to the +input of the opamp and put the
switch between
the output and the -input and VCC. In one position the mux would make
the opamp act as a follower, in the other the inverting input's voltage
would
be much higher than the non-inverting one, so the open loop gain would
make the output saturate somewhere close to VEE.
Other requirements and don't cares:
- the switching frequency between the DAC voltage (VDAC) and the
negative output voltage (VNEG) is negligible (<10Hz).
- the transition time between VDAC ad VNEG should be as fast as possible.
- the transition time between VNEG and VDAC is not important, but
the VOUT should have no significant overshoot above VDAC.
In other words, the slow path must be precise and the negative
VOUT path must be fast.
I'm tempted to use the solution number 3, but I'm afraid of the
open-loop connection. Or should I do it in a completely different way?
Best regards, Piotr
TL;DR: a precision linear MOSFET pre-driver capable of putting negative
voltage on the gate in order to shut the current flow down. Quickly and
hard to eliminate spurious noise-induced turn-ons.
Detailed:
I have a precision unbuffered 16-bit DAC that outputs 0--4.095V. The
analog subsystem uses +12/-5V supplies in order to place the opamps
(OPA4192)
well within the linear region. Now I would like to buffer the VDAC
voltage, but also augment the buffer with the ability to output some
negative VNEG voltage in shutdown state. The closer VNEG is to the VEE
rail the better, but the exact value is unimportant. I can see 3
possible scenarios based on a DG419 SPDT switch:
1. Create a follower and add the mux to the input, switching between
VDAC and VEE. But this can add noise/nonlinearities to the precision path.
2. As above, but put the mux at the follower's output. This would add me
~30 Ohms to the output impedance.
3. Connect the DAC directly to the +input of the opamp and put the
switch between
the output and the -input and VCC. In one position the mux would make
the opamp act as a follower, in the other the inverting input's voltage
would
be much higher than the non-inverting one, so the open loop gain would
make the output saturate somewhere close to VEE.
Other requirements and don't cares:
- the switching frequency between the DAC voltage (VDAC) and the
negative output voltage (VNEG) is negligible (<10Hz).
- the transition time between VDAC ad VNEG should be as fast as possible.
- the transition time between VNEG and VDAC is not important, but
the VOUT should have no significant overshoot above VDAC.
In other words, the slow path must be precise and the negative
VOUT path must be fast.
I'm tempted to use the solution number 3, but I'm afraid of the
open-loop connection. Or should I do it in a completely different way?
Best regards, Piotr