On 5/9/08, Martin wrote: > Vasile Surducan wrote: > > On 5/8/08, Mark Rages wrote: > >> On Thu, May 8, 2008 at 11:45 AM, Martin wrote: > >>> I have an analog circuit that has a (normally) negative polarity DC > >>> feedback signal. I'd like to change the gain of the circuit digitally, > >>> so I'd like to use a digital pot. The problem is that a digital pot can > >>> only take signals between VEE and VDD - most usually probably 0v to 5v. > >>> > >>> My rails are +5/0/-5 and my feedback lies somewhere between 0 and -5. > >>> Has anyone encountered a digital pot that can either take a 10v supply, > >>> or has level shifters on the digital lines? I think I might be able to > >>> use a CD4052 with discrete resistors, but then it's a 16 pin chip I have > >>> to put on the board. > >>> > >>> Any ideas? > >>> > >> I assume the digital lines are 0->+5 V. > > > > If he is designing the whole stuff, you may assume digital lines are 0 > > -5V (if it's a clever designer) so he don't need any level shifter. > > > > Vasle > > > > If I'm clever, eh? First product I designed, I'm the only engineer > working on it - I'm lucky it works at all. Digital has to be ground > referenced. > > The input opamp is an LMC6042AIN. I need the extremely low input bias > current spec. so I can't use a controlled-gain amplifier i.e. LTC6910. > > I found an SPDT analog switch in SSOP8: TC7W53 > > > Should give me the gain range I need. Martin, this is not different from any other analogic switch except maybe the two channels... There is a mistake on the first page of the datasheet, for switching analogic signals between -5V to +5V you need Vcc=+5V and Vee=-5V. On the other hand, if you really need inputs of 2-5fA then TAKE CARE about any digital feedback into the analogic path. Working alone is easier than working in a team because all faults are just yours...you can't blame anybody and no one will blame you... except the customer if there is any. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist