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Precision Data Converters
Precision Data Converters Forum
Hi my name is Ralph Reiter, i am working at the Luxembourg University and i am exploring some of my free time with your high end precision dataconverters.
Now i bought the ADS1282 EVM Board and i saw in the description document that i need to supply also a + and - 10V.
In the schematic it seems to serve only as a supply for the input buffering opamps. My question is :
Can i also use the ADS1282 EVM with + - 9 V on these 10 V pins. For me i think it would be ok???
Thank you in advance for your answer.
+/-9V supplies should work on the ADS1282EVM board. Those supplies should be more than enough for the references (REF5050 and REF5025), reference buffer (OPA227) and the signal input amplifiers (OPA1632).
I was hoping to be able to power the ADS1282-EVM (stand alone) with a 12V supply, without doing any DC/DC to get a larger +/- voltage range. Is it possible to split the 12V to +/-6V and use +/- 5V for analog supply and +/-6V for the opamp supply (OPA1632) - specified to +/- 10V in datasheet of the EVM?
Best regards, Gaute
The OPA1632 may be powered with a lower supply of +/-6V (connected at +VA/-VA on J8 pin1 and pin2) on the ADS1282EVM stand alone . You will need to generate and feed the positive +6V and negative -6V supplies; using a DC to DC converter. You may also power the OPA1632 with a +/-5V supply (connected at +VA/-VA on J8 pin1 and pin2).
In this case, the user is restricted to use the ADS1282 in bipolar supply mode. In order to configure the ADS1282 converter device on the ADS1282EVM stand alone with +/-2.5V bipolar supplies, you will need to feed +/-5V analog supplies into J8 pins 3 and 4; and ensure GPIO3 on J5-pin12 is pulled low. This will enable the +/-2.5V voltage regulators (U10 and U11) and set up the ADS1282 with a bipolar supply +/-2.5V.
When reducing the supplies of the OPA1632, the user needs to be concerned with the reduced input common-mode voltage range and the reduced output swing of the OPA1632. Figure 9 on the OPA1632 shows a plot of the output swing of the OPA1632 with RF=1kOhm. See Fig 9 below.
NOTE: In the case where the user is feeding +/-5V to J8 pin1/pin2 instead of +/-10V to the ADS1282EVM stand alone; the ADS1282EVM stand alone is restricted to be used in bipolar supply mode by setting J5-pin12 low (GPIO3). Otherwise, other circuits on the EVM will not have enough headroom .
If I am correct the -5VA supply for the ADS1282EVM should be able to handle -6V (regulated down to -2.5V in bipolar mode by TPS72325). But can I power the +5VA with +6V (the TPS79225 mentions Vmax = 5.5V, but 6V in maximum ratings)?
The TPS72325 will operate with negative voltage VIN -6V as you mentioned; since this is inside the allowed operating voltage guaranteed on the datasheet.
However, the TPS79225 maximum operating input voltage spec is 5.5V. The +6V absolute maximum rating on the TPS79225 refers to a fault or stress condition; where in a fault condition , the device will tolerate a maximum input voltage of +6V without causing permanent damage to the device; however, +6V is not inside the normal operational voltage range. The allowed operational input voltage range for the TPS79225 is from +2.7V to +5.5V.
The user may operate the ADS1282EVM with a voltage of +5V to +5VA on the ADS1282EVM in unipolar mode; the maximum operating voltage of the ADS1282 is 5.25. In addition, there is a 5.1V zener diode (D8) between AVDD and AVSS.
In bi-polar supply mode, the TPS79225 should only operate up to +5.5 AVDD.
Thanks for your reply.
Would supplying the TPS79225 with +5V on the ADS1282EVM +5VA terminal from a switching regulator like the LM2674 N-5.0 be a bad idea? If I have understood correctly the TPS79225 on the ADS1282EVM along with its input and output capacitors should be able to suppress most the input noise.
The TPS79225 is a High PSRR linear regulator. Linear regulators have low noise at their output; therefore, they may be connected directly to the supply pins of the ADS1282EVM with the appropriate bypass capacitors. When using switching DC-DC converters such as the LM2674, it is recommended to cascade a linear regulator with high PSRR in front of the DC to DC converter in order to attenuate the ripple of the DC to DC converter.
Attached below is an application note that showcases different DC-DC converters and Linear regulators that you could combine to produce a low noise voltage supply as required for high resolution ADC applications
If you are planning to design a PCB board with the ADS1282 and the DC-DC converter, it is also important to consider isolating the ADS1282 from the DC-DC converter by using a split ground plane and keeping the DC-DC converter away from the sensitive analog signals.
Thanks for your help and patience, you have been of great help,
I am looking at the power consumtion of the ADS1282EVM and I notice that the +/- 10 VA inputs draw ~ 30 mA, this is reasonable as the 2x OPA1632 should need about that (is there a way to disable the second OPA1632?). However I notice that DVDD draws ~ 60 mA, I have not hardwired any of the digital inputs (PDWN, M*, etc..) and I am using U7/PCA9535 to configure these pins. The ADS1282 itself should not need anything close to this amount of current, is all this going into the U7 or is there anything I am missing?
If it goes into the U7; all of its outputs are either pulled up or down to a default, I have configured all pins I do not change as inputs on the U7 hoping that this would result in a minimum of current draw, am I wrong to assume this?
Also; it seems I cannot have different Agnd and Dgnd on the EVM, is this correct?
The +/-10VA is feeding the (2) OPA1632s, the OPA227 and the REF5050 and REF5025. A current of ~30mA is reasonable. The OPA1632's are wired in, so there is no way to disconnect them other than physically removing them or modifying the EVM.
The consumption I obtain through the DVDD supplies is in the order of ~2mA; the DVDD pin of the ADS1282 should consume current in the order of 800uA and the PCA9535 should also consume low current. I measured the current on the J7 jumper on the ADS1282EVM top board. I am not sure why your result is 60mA. Where are you measuring the current?
The AGND and DGND are shorted together in the EVM board, but the board layout splits the analog ground plane below the traces routing all the analog signals and a digital ground plane below the digital interface connections and crystal.
Luis ChioyeThe consumption I obtain through the DVDD supplies is in the order of ~2mA; the DVDD pin of the ADS1282 should consume current in the order of 800uA and the PCA9535 should also consume low current. I measured the current on the J7 jumper on the ADS1282EVM top board. I am not sure why your result is 60mA. Where are you measuring the current?
I measured this through the line going into J8.9, 3.3V. I get ~ 2mA before operation or setup, but after having configured the PCA9535 and running at full power with continuous sampling (250Hz) it rises to ~ 60 mA.
I have additionally connected: +10VA, -10VA, +5VA, -5VA and Agnd/Dgnd with all jumpers on J7 on.
I configured all of PCA9535 ports as inputs (omitting an, apparently redundant, RESET during startup) and now my DVDD consumption is down to ~6 mA.
Edit: I had to keep doing the RESET, but with all controlregisters of U7 set to 0xFF and 0xFF.. I don't know why it works, but the consumption dropped and the EVM initializes apparently correctly. Perhaps it is a required time delay.
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