Hi I need to sample two accelerometers and two gyroscopes with the MSP430F149's ADC. However, they output an analog signal in the 0-5V range. So I have to lower it to a 0-2.8V range in order to interface it with the MCU. I was wondering if it was safe to use a voltage divisor to do this (2 resistors), or should I use something more complicated like some kind of amplifier with 0.56 (2.8/5) voltage gain in order to isolate the MCU load. I was thinking that since the MCU has a very high input impedance the voltage divisor should be ok, but I'm not sure. Any suggestions? Thanks, Carlos.
Connecting Sensors to MSP430
Started by ●May 15, 2005
Reply by ●May 15, 20052005-05-15
Voltage divider is ok. 10k impedance is a good choice. With higher
impedance
you need to make your sample interval larger. (there is a formula in the
datasheet to calculate the sample timing for different input impedances.)
Matthias
On Sun, May 15, 2005 at 08:34:35PM -0000, elcoqui00
wrote:
> Hi
>
> I need to sample two accelerometers and two gyroscopes with the
> MSP430F149's ADC. However, they output an analog signal in the 0-5V
> range. So I have to lower it to a 0-2.8V range in order to interface
> it with the MCU. I was wondering if it was safe to use a voltage
> divisor to do this (2 resistors), or should I use something more
> complicated like some kind of amplifier with 0.56 (2.8/5) voltage gain
> in order to isolate the MCU load. I was thinking that since the MCU
> has a very high input impedance the voltage divisor should be ok, but
> I'm not sure.
>
> Any suggestions?
>
> Thanks,
> Carlos.
>
>
>
>
> .
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
Matthias
Reply by ●May 15, 20052005-05-15
By 10k do you mean the parallel of the two resistors in the voltage divider? --- In msp430@msp4..., Matthias Weingart <msp430@p...> wrote: > Voltage divider is ok. 10k impedance is a good choice. With higher impedance > you need to make your sample interval larger. (there is a formula in the > datasheet to calculate the sample timing for different input impedances.) > > Matthias > > On Sun, May 15, 2005 at 08:34:35PM -0000, elcoqui00 wrote: > > Hi > > > > I need to sample two accelerometers and two gyroscopes with the > > MSP430F149's ADC. However, they output an analog signal in the 0- 5V > > range. So I have to lower it to a 0-2.8V range in order to interface > > it with the MCU. I was wondering if it was safe to use a voltage > > divisor to do this (2 resistors), or should I use something more > > complicated like some kind of amplifier with 0.56 (2.8/5) voltage gain > > in order to isolate the MCU load. I was thinking that since the MCU > > has a very high input impedance the voltage divisor should be ok, but > > I'm not sure. > > > > Any suggestions? > > > > Thanks, > > Carlos. > > > > > > > > > > . > > > > > > Yahoo! Groups Links > > > > > > > > > > > > > Matthias
Reply by ●May 16, 20052005-05-16
10k resistors (2 of them) to divide the voltage down. Keeps the source impedance low for the ADC, whose input impedance is not high, only about 2k. Note, this should be compared against the output drive capability of the sensor. Some sensors are only capable of driving loads of 100k or more which would require a voltage divider and an op-amp. Be careful with your op-amp idea, input voltage range is usually about 1V below rail. So, if your op-amp has the same supply as your sensors you'll lose the high end portion to input clipping on the op-amp. Attenuate before op-amp then gain back up to unity. Alan ----- Original Message ----- From: elcoqui00 To: msp430@msp4... Sent: Sunday, May 15, 2005 5:31 PM Subject: [msp430] Re: Connecting Sensors to MSP430 By 10k do you mean the parallel of the two resistors in the voltage divider? --- In msp430@msp4..., Matthias Weingart <msp430@p...> wrote: > Voltage divider is ok. 10k impedance is a good choice. With higher impedance > you need to make your sample interval larger. (there is a formula in the > datasheet to calculate the sample timing for different input impedances.) > > Matthias > > On Sun, May 15, 2005 at 08:34:35PM -0000, elcoqui00 wrote: > > Hi > > > > I need to sample two accelerometers and two gyroscopes with the > > MSP430F149's ADC. However, they output an analog signal in the 0- 5V > > range. So I have to lower it to a 0-2.8V range in order to interface > > it with the MCU. I was wondering if it was safe to use a voltage > > divisor to do this (2 resistors), or should I use something more > > complicated like some kind of amplifier with 0.56 (2.8/5) voltage gain > > in order to isolate the MCU load. I was thinking that since the MCU > > has a very high input impedance the voltage divisor should be ok, but > > I'm not sure. > > > > Any suggestions? > > > > Thanks, > > Carlos. > > > > > > > > > > . > > > > > > .
Reply by ●May 16, 20052005-05-16
Thanks! The sensors can drive around 2 mA, so they should be ok. --- In msp430@msp4..., "Alan Zubatch" <zubatch@a...> wrote: > 10k resistors (2 of them) to divide the voltage down. Keeps the source impedance low for the ADC, whose input impedance is not high, only about 2k. Note, this should be compared against the output drive capability of the sensor. Some sensors are only capable of driving loads of 100k or more which would require a voltage divider and an op-amp. > > Be careful with your op-amp idea, input voltage range is usually about 1V below rail. So, if your op-amp has the same supply as your sensors you'll lose the high end portion to input clipping on the op-amp. Attenuate before op-amp then gain back up to unity. > > Alan > > ----- Original Message ----- > From: elcoqui00 > To: msp430@msp4... > Sent: Sunday, May 15, 2005 5:31 PM > Subject: [msp430] Re: Connecting Sensors to MSP430 > > > > By 10k do you mean the parallel of the two resistors in the voltage > divider? > > --- In msp430@msp4..., Matthias Weingart <msp430@p...> wrote: > > Voltage divider is ok. 10k impedance is a good choice. With higher > impedance > > you need to make your sample interval larger. (there is a formula > in the > > datasheet to calculate the sample timing for different input > impedances.) > > > > Matthias > > > > On Sun, May 15, 2005 at 08:34:35PM -0000, elcoqui00 wrote: > > > Hi > > > > > > I need to sample two accelerometers and two gyroscopes with the > > > MSP430F149's ADC. However, they output an analog signal in the 0- > 5V > > > range. So I have to lower it to a 0-2.8V range in order to > interface > > > it with the MCU. I was wondering if it was safe to use a voltage > > > divisor to do this (2 resistors), or should I use something more > > > complicated like some kind of amplifier with 0.56 (2.8/5) > voltage gain > > > in order to isolate the MCU load. I was thinking that since the > MCU > > > has a very high input impedance the voltage divisor should be > ok, but > > > I'm not sure. > > > > > > Any suggestions? > > > > > > Thanks, > > > Carlos. > > > > > > > > > > > > > > > . > > > > > > > > > Yahoo! Groups Links > > > > > > > > > > > > > > > > > > > > Matthias > > > > > . > > > > > > ------ > Yahoo! Groups Links > > a.. To visit your group on the web, go to: > http://groups.yahoo.com/group/msp430/ > > b.. . > > c.. Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service. > > > >
Reply by ●May 16, 20052005-05-16
--- In msp430@msp4..., "elcoqui00" <crperez@g...> wrote: The voltage divider should be fine. However, if your concerned about protecting the MSP. You might what to use optoisolators as inputs to the MSP. > Hi > > I need to sample two accelerometers and two gyroscopes with the > MSP430F149's ADC. However, they output an analog signal in the 0-5V > range. So I have to lower it to a 0-2.8V range in order to interface > it with the MCU. I was wondering if it was safe to use a voltage > divisor to do this (2 resistors), or should I use something more > complicated like some kind of amplifier with 0.56 (2.8/5) voltage gain > in order to isolate the MCU load. I was thinking that since the MCU > has a very high input impedance the voltage divisor should be ok, but > I'm not sure. > > Any suggestions? > > Thanks, > Carlos.
