Arduino control peristaltic pump
Whoa, no wonder you're burning these:




Absolute Maximum Ratings





Voltage on VDD with respect to VSS
-0.6V to +7.0V


Voltage on /CS, SCK, SDI, SDI/SDO, and /SHDN with respect to VSS
-0.6V to 12.5V


Voltage on all other pins (PxA, PxW, PxB, and SDO) with respect to VSS
-0.3V to VDD+0.3V




Essentially, you're feeding -40 and -50 volts to the pins PxA, PxW and PxB. Their absolute limit is rated at -0.3, up to VDD+0.3 volts. VDD can be up to 7 volts without damage, but the typical operating voltage is 5 volts.
There are a number digital potentiometers that can handle 10 volts, but you must make sure that you float them, if the pins you need to use really are at a potential of -40 or -50 volts. This is not necessarily easy to do.
Another alternative are motorized potentiometers.
You may get more solutions if you try to post the current schematic, maybe there's a way to bypass the potentiometer completely.