AD5206 w/ programmable gain amp minimum current/load issues? - Page 1
Forum section: electronics / projects

author: johm
author_rank: Contributor
timestamp: February 05, 2015, 08:37:00 am
content: Good day to everyoneI've got an LM324 op amp in an inverting (x2 - x0 NOT linearly interpolated) configuration. With constant 5V as an input, its gain is programmable using an AD5206 100k digital pot. The output from the op amp is then phase inverted. The problem is I'm not getting full swing output from 0-ish V (offset voltage and AD5206 nuances) up to +10V. I get 196mV to 492mV, LTSpice simulation looks OK (0V - 10V), does anyone know what I'm missing here?- It's not a blown chip issue. (changed both AD5206 and LM324)- SPI Bus is OK- AD5206 voltage divider configuration works with an LED (as inhttp://arduino.cc/en/Tutorial/SPIDigitalPot)- LM324 inverting circuit works with 100k old-school linear potentiomenter.- All voltages match LTSpice simulation except voltage at output of LM324.I'm sure it's some RTFDatasheet thing but after having bed-time-read the datasheet over and over again my knowledge is limited.Don't be shy to have a glance at the schematic.

author: Kalvin
author_rank: Super Contributor
timestamp: February 05, 2015, 09:30:58 am
content: it is not shown in the schematics how you have connected the AD5206.I guess that you have connected the chip so that VSS and GND are connected to 0V and VDD to +5V. If these assumptions are correct, the circuit will behave as you have noticed.The first op amp U1 is connected as an inverting amplifier, so U1 will want to pull its output to the negative voltage as you are feeding positive current to its input. As the AD5206's B (R4) is connected to U1's output, the B's right hand side will be pulled to negative voltage. However, as the VSS is connected to the 0V, none of the AD5206's input cannot swing to the negative voltage - instead the protection diodes will be clamping the voltage to -0.1V ... -0.4V as you are seeing now.I would change the design to a non-inverting topology. In this non-inverting configuration I would first make a resistive 1:2 voltage divider using two 50K resistors. The voltage divider would feed a non-inverting op amp used as a unity gain buffer: the op amp's output will stay between 0V - 2.5V as the input voltage swings between 0 - 5V. This op amp would drive the AD5206's A, while the AD5206's B is connected to GND. The AD5206's wiper W would then feed another non-onverting op amp with a gain of 4.Edit: This non-inverting topology will not implement the original gain as a function of AD5206's wiper value.If you do want to keep your original topology, you should connect the AD5206's VSS to a negative voltage (say -2.5V or -1V) and make sure that the voltage difference between VDD and VSS is not more than 5V, as stated in datasheet. You should then limit the U1's output voltage swing to VSS (-2.5V or -1V) . So, I would still add the resistive 1:2 or 1:5 voltage divider and the buffer op amp with a unity gain. This unity gain op amp would then feed the input resistor R1 55.2 kohm. Finally, the U2's gain should be increased from -1 to -2 or -5, so the R6 should be increased to 18kohm or 45kohm.Br,Kalvin

author: johm
author_rank: Contributor
timestamp: February 05, 2015, 03:26:15 pm
content: Thanks for your reply Kalvin, your assumptions were right. Also I'm interested on having a linear (if possible) dependance with respect AD5206's wiper. Your answer encouraged me again and new options resurfaced. Because going split-supply is too much of a trouble, I'll go back to square one and use your non-inverting dual op amp solution and try to figure out how to implement this dependance over AD5206's wiper.Regards,Johm.