AD7794 undocumented gain settings

Question I'm interested in additional register information to the configuration register of the AD7194. It has 3 Gain bits to select the ADC input range. Is it possible to also use Gain 2 or 4 (I assume a value of 1 or 2 to write to the gain bits). In the current datasheet these bit setting is marked as reserved. Answer You are correct – if the gain bits are set to 1, a gain of 2 is selected while a gain of 4 is selected if the gain bits are set to 2. However, when we evaluated the part prior to release, we found that the INL breaks the datasheet spec at a gain of 2 and 4. Therefore, we removed these gains i.e. they are not documented in the datasheet. You can use these gains. However, specs are not guaranteed for the part at these gains. For this reason and also because you are designing a precise system, it may be better to use one of the documented gains.

AD7794 evaluation software sample limitation

Question In Evaluation Board for the AD7794 (24-Bit, 6 Channels), when I use the AD PC software, what is upper limitation of sample size, that has to be entered in the "Num Samples" text box? Answer The software can gather up to 65000 samples.

AD7794: Hot to use the Eval Board standalone

Question I am using the EVAL-AD7794/95EB board, and I can't setup a SPI communication with using an STR9-EVAL board , I am using the external power supply (5V) LED is ON (D1 is on when USB connected), I would like to know if ADC's SPI can be used or if it is only for USB communication. Answer With the USB connection in place, the microcontroller on the eval board is powered up and will 'talk' to the ADC. To use the board as a stand-alone, an external power supply should be applied to the board, the USB cable should be disconnected and the tracks from the on-board micro to the ADC would need to be cut. There are some test points on the board which allow the user to access the serial interface lines. However, the lines are presently connected to the microcontroller's interface.

Measuring DC offset in an audio signal

Question I need to measure the DC offset of an audio signal at various points in a circuit. The signal amplitude can vary between +-12V. Ideally, I would like to be able to measure offsets with a precision of a few micro volts. I was thinking of using an ADC like AD7794 to get the resolution and to to have a low inherent offset, driven by an ADA4528-2 to get the correct amplitude at the input of the ADC (between 0 and 5V, with no signal = 2.5V) To get the best resolution using AD7794, I need to sample at about 4Hz. I gather that I therefore need an antialiasing filter at around 2Hz or lower, which would prove very difficult to implement. Needing the resistor values of the filter to remain low ( because of the Johnson noise), I would need massive capacitors, very impractical. Do I really need to filter prior to the AD7794 (audio bandwidth is several 100kHz)? What would happen to the DC part of the signal if there was no filter? Another method I can think of is to use an audio delta sigma ADC, measure the whole audio bandwidth and filter in the digital domain (in a DSP), but these audio ADC usually have poor DC offset characteristics, making it I believe impossible to get an accurate DC offset measurement. My last idea is to use an SAR ADC, but I am so far stuck with the resolution of the SAR (16 bits max usually). With 5V Full scale, a 16 bit step is about 75uV and I would ideally like a better accuracy. Would averaging in a DSP give me a much better accuracy, I am not sure. Heavily filtering before the ADC and I am stuck with the same problem as before. Do you see other methods to accurately measure the DC offset of such an audio signal? Am I making a mistake in my 3 ideas above? Please bear in mind that the ADC output will be connected to a 400 series Sharc DSP, so there is some filtering / computing power available. Your views are greatly appreciated. Answer The attached Excel file shows the filter response of the AD7794 for an output data rate of 4Hz. You should add the standard RC anti-aliasing filter that we recommend which provides rejection at the modulator sampling frequency. This anti-aliasing filter along with the AD7794's digital filter should reject the audio tone and leave the DC signal. I do not think that a filter with a 3dB point of a few Hz is needed. I have never tried this method before so I am not guaranteeing it. However, in principle, it should work.

AD7794 internal short for debug

Question I have a question regarding the AD7794 datasheet. On page 23 there is a table of channel select bits. For an input of 1000, the channel is states as AIN1(-)/AIN1(-). This will always be zero, so I wondered whether it is correct, or whether it should read AIN1(-) /GND. Please could you clarify? Answer The AD7794 datasheet is correct. This channel is an internal short. It is useful for debugging. If e.g. you have noise issues with your system, we sometimes suggest that you use this internal short for debug. This helps to locate the noise source.

AD7794: NOXREF active voltage and temperature sensor calibration

Question I've got the latest datasheet but there are still two unresolved problems: question 1: page 5 says, NOXREF is active when Vref is less than 0.3 V but page 20 tells me 0.5 V. What is the real low limit for missing reference detection ? question 2: Internal Temp Sensor, how to calculate T from the voltage (I've seen a value of 0.81 mV per degree) Must I calibrate this measurement overwriting the offset resp. zero registers used for channel 4-6 (AIN 4 is already in use with a different external reference and gain = 1) ? Answer Question 1: Page 20 is correct, NOXREF becomes active when the reference voltage goes below 0.5V. Question 2: The 0.81mV/degC is a sensitivity specification so you need to calibrate the sensor at some known temperature. To calibrate the temp sensor, you need to read the conversions from the device at a known temperature, for example, 0 deg C. At 0deg C, the temp sensor should generate a code of 000000. You can then record the value generated by the temp sensor at 0 deg C. This value can be held in the uC and subtracted from each conversion read back from the AD7794. Channels 4-6 share the same offset and gain registers. You can select each channel in turn and perform a calibration. The offset and gain values could then be stored in the uC. Each time the channel is changed, the appropriate offset and gain values could be written to the offset and gain registers on the AD7794. The other alternative is to perform a calibration each time the channel is changed. However, this would require more time.

Is there any way of getting the firmware in the microcontroller to control the SPI communication (SPI-USB Bridge)?

Q Is there any way of getting the firmware in the microcontroller to control the SPI communication (SPI-USB Bridge)? A We cannot supply the source code for the firmware. This is a canned routine that we use. We have the LabView source code and we can send this to you. However, we do not have the uC source code i.e. the code that controls the interaction between the uC/ ADC and the uC/USB. We have a hex version of this code which is called within the LabView code. Attached is some sample C code which may help. It is code for the interaction between the ADC and the uC. It was written for the AD7792 which is a 16-bit part but it still may be useful to you. Also attached is the labview source code. It was written in Labview 8.2. Attachments: ExampleCode_ADC_AD7792.zip AD7794-Rev2-3-llb.zip

AD7794: RMS noise spec

Q There is an information in AD7794 Data Sheet: RMS noise: 80 nV (Gain = 64). And there is an information in Product Description and presentation about AD779x: 40 nV (Gain = 64). What is true? A The rms noise is 80nV when the gain equals 64 and the update rate is 16.7 Hz (default value). When the update rate is reduced to 4.17Hz, the rms noise is reduced to 40nV with the gain at 64.

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AD7794 Production
The AD7794/AD7795 are low power, low noise, complete analog front ends for high precision measurement applications. They contain a low noise, 24-/16-bit...
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