Hi.
Working on yet another PIC development board (this time for analogue) I've ran into the age old problem of getting a full range out of an opamp. I have a 10-bit DAC on board, MCP4812, and altohugh its output is buffered, I would like to add a voltage follower on the output, so I can draw just a little bit more current from it before it's error becomes too much.
For ease of use, an LM358 springs to mind. My board will be powered from 3.3/5V (selectable by user) so, in order to increase the rail output, I'm happy to use a charge pump voltage doubler, to provide the opamp with ~6/10V. But, given the resolution of the DAC, at the lower end I will never get true 0V out, even the LM358 bottoms out at 40mV... which is a few LSB's! The top end sohuldn't be too much of a problem, as the DAC has an onboard 4.096/2.048V reference, which means that is the DACs full scale output. I could of course regulate the doubled voltage to 5V for the DAC, so it can output 4.096V when running at 3.3V. Or just switch the voltage down to 2.048V.
So, a negative supply is required, albeit a very small one ~ -1V.
As you can see, its starting to all add up. Normally I would just use the +V and -V outputs from a MAX232, but I do not have one of these in my design. Not only is its quiescent current quite high for a 'low power' system, but also, many only run at 5v, or 3.3V, not both. I'm trying to avoid having two seperate voltage converters, one for double Vcc, and one for -Vcc. Only solution I have so far is the MAX680, but that runs at 8kHz (bit too low, bit noisey eh?) and its current output is dreadful. Another option is a dual charge pump with a 555 timer. However, the NE555 doesn't run <4V, and its CMOS equivilent has a very limited current output (crap for charge pumps).
So, any idea's for simplicity? I realise rail-to-rail opamps are available, but these are never true rail to rail. Also a half decent current output is needed, we'll say 10mA+ for the positive supply, so I can draw *some* current from the opamp buffer, without it sagging.
Once again, I'm over complicating things, since the problems with the negative rail only occur for <100mV for the LM358 (with a current load). But at 10-bit, with a Vref of 2.048, thats 50 LSB's out.
Blueteeth
Working on yet another PIC development board (this time for analogue) I've ran into the age old problem of getting a full range out of an opamp. I have a 10-bit DAC on board, MCP4812, and altohugh its output is buffered, I would like to add a voltage follower on the output, so I can draw just a little bit more current from it before it's error becomes too much.
For ease of use, an LM358 springs to mind. My board will be powered from 3.3/5V (selectable by user) so, in order to increase the rail output, I'm happy to use a charge pump voltage doubler, to provide the opamp with ~6/10V. But, given the resolution of the DAC, at the lower end I will never get true 0V out, even the LM358 bottoms out at 40mV... which is a few LSB's! The top end sohuldn't be too much of a problem, as the DAC has an onboard 4.096/2.048V reference, which means that is the DACs full scale output. I could of course regulate the doubled voltage to 5V for the DAC, so it can output 4.096V when running at 3.3V. Or just switch the voltage down to 2.048V.
So, a negative supply is required, albeit a very small one ~ -1V.
As you can see, its starting to all add up. Normally I would just use the +V and -V outputs from a MAX232, but I do not have one of these in my design. Not only is its quiescent current quite high for a 'low power' system, but also, many only run at 5v, or 3.3V, not both. I'm trying to avoid having two seperate voltage converters, one for double Vcc, and one for -Vcc. Only solution I have so far is the MAX680, but that runs at 8kHz (bit too low, bit noisey eh?) and its current output is dreadful. Another option is a dual charge pump with a 555 timer. However, the NE555 doesn't run <4V, and its CMOS equivilent has a very limited current output (crap for charge pumps).
So, any idea's for simplicity? I realise rail-to-rail opamps are available, but these are never true rail to rail. Also a half decent current output is needed, we'll say 10mA+ for the positive supply, so I can draw *some* current from the opamp buffer, without it sagging.
Once again, I'm over complicating things, since the problems with the negative rail only occur for <100mV for the LM358 (with a current load). But at 10-bit, with a Vref of 2.048, thats 50 LSB's out.
Blueteeth
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