circuit for starting with a fixed negative voltage and then adjusting it (DAC) - Page 1
Forum section: electronics / projects

author: CopperCone
author_rank: Super Contributor
timestamp: September 15, 2017, 02:02:16 am
content: What kind of circuit could be used (that works unconditionally) which will start with a fixed negative voltage (-1V) and then afterwords (when other sequencing steps are completed) can be adjusted (to set current) using a measuring circuit? It is expected that it will be adjusted between -0.9 and -0.6V with about 8 bits of resolution.I worked out the measuring circuit, the output of which can be a voltage or current proportional to the current draw from the rail.Is there a negative (low noise) linear regulator that will work with some kind of a "tap" inserted into its adjust pin?And the adjustment circuit is analog (this cannot change) meaning that the rail can only be adjusted (with something that is switched in using relays or transistors, don't care), while not being significantly perturbed during the the adjustment circuit connection (I guess having significant output capacitance can be a plus here).I wanted to use low noise LDO, such as the LT3090. I know how to initially set it to -1V, but how can a control signal be tapped into it?I don't want to use the DAC to initially set the thing, then adjust it, I need it initially set by something fixed, so the weight of the DAC control signal is small in proportion to the 'hard' setting.Since its 50uA, could I just feed a small signal in there that is switched in with a transistor? It would be very high impedance and noise sensitive, but I don't see another way..Use two resistors for the set current and put a resistor T in the middle of them?

author: Rerouter
author_rank: Super Contributor
timestamp: September 15, 2017, 10:22:13 am
content: 2 dacs. One fine one main. The main one sets your setpoint. And the other fine tunes it. If you have a -1v rail already available you would connect the second dacs vref to ground to give you a full scale control of 0 to -1v, to meet your resolution you would then use a 10 or 12 bit dac, the microchip ones are dirt cheap with an output buffer.

author: danadak
author_rank: Super Contributor
timestamp: September 15, 2017, 12:05:09 pm
content: If your control side, DAC, A/D, on positive supply rail you could handle theDAC output by using a V dac driving into a negative referenced currentmirror which in turn adds current to the set R according to DAC value. The groundof the mirror is the negative rail of the system.The A/D interface, to measure the - V, could be as simple as two R's to offsetthe voltage into the A/D input on the positive rail. This has an ap note and acalculator to do that -https://www.dropbox.com/s/k1yn4m7heaixvmg/ADinput%20negative%20inputs.zip?dl=0A PSOC could handle this (it has an internal reference as well), see attached. Note youhave to design the output of the IDAC (turned into VDAC) to comply with the CM rangeallowed at OpAmp output (GND in this case).Regards, Dana.

author: David Hess
author_rank: Super Contributor
timestamp: September 15, 2017, 04:08:16 pm
content: Quote from: CopperCone on September 15, 2017, 02:02:16 amSure, it is called an operational amplifier combined with an output transistor if higher current is needed. The transistor may be replaced with a low voltage negative regulator if output protection is required.If a positive reference voltage is already available, then that can be used instead of a separate negative reference by configuration the operational amplifier as an inverting amplifier.This is a lower noise solution than any integrated regulator alone.