Other Parts Discussed in Thread: TPS7A33,
Hi,
I have a problem with my MMIC bias circuit using an AMC7834 for the analogue parts.
The problem is around the negative supply rail and the Avss clamp function of the bipolar DACs in the AMC7834. If the negative supply is not present, the negative rail seems to be internally pulled up to about +0.4V (internal clamp diode?), taking the output of the bipolar DACs with it through the Avss clamp function. As my intended MMICs are very sensitive, this is not acceptable.
Please see the measurement below for clarification:
Now, I guess this is somewhat predicted behavior with the bipolar DACs, which only clamp to the clamp pin voltage (connected to Gnd) if the device is operating with all bias rails active, and otherwise to AVSS as far as I understand it.
The datasheet states that no specific power supply sequencing is required, however it looks like that the pull up to +0.4V is mainly due to the AMC itself - there's no external circuitry on the VSS rail, except for the LDO (ADP7182, weak 220kOhm pull to GND when powered down).
I have tried different things to bypass this - activating clamp manually (trying to force using the clamp pin voltage rather than AVss clamp) rather than relying on the AVSS alarm (with the Avss clamp), a simple pulldown to Gnd but it doesn't help the situation.
The only option I see right now to prevent this is to use a relais or equivalent transistor circuit to force AVss to ground if the external supply is not operating. As this requires a hardware change I'd be interested in other options, is there anything I haven't seen yet? Is this ~0.4V pull up standard for the AMC7834?
A different option would be to use a negative regulator which clamps the output strongly to ground when deactivated. The TPS7A33 used in the AMC7834EVM doesn't seem to do that either, so I'd expect the EVM to show a similar behaviour to my board. I have tested a 500 Ohm resistor between VSS and ground, that doesn't help the situation. Having a GND clamp option for the bipolar DACs would be great...the AVSS clamp seems problematic.
My AMC7834 is a version id 0 chip - bought recently from a major retailer. Could this be part of the problem?
The changelog in the datasheet doesn't seem to indicate so...
The AMC configuration is: bipolar DAC range 0-+5V, PA on, clamp deactivated (both tested), AVss alarm activated (both tested), open loop operation. DAC values set to 0.
Thanks,
Kai
*edit* to re-insert image that was missing.