Bipolar input version of AD7812

Question I'm looking for an equivalent part to AD7812 but one which can accept a +/- input. Preferably pin compatible. Answer The analog input channels can be configured as pseudo differential channels..refer to page 9 of the datasheet for more details. We don’t have bipolar input range ADC‘s in the 10 bit space. The closest to the AD7812 would be the parts that are available in the general purpose 100KSPS to 1MEGSPS 12 bit resolution. Refer to the selection guide on analog.com for a full listing.

AD7812: Control and addressing

Question 1) In the manual there is an information that this IC has 10 bit control register, but the timing diagram shows that I have to write 0x6040 ( 16 bit ) after power-up and 0x4040 every cycle. Is it a mistake in manual or how can I write 16 bit in 10 bit register? 2) As I realised, it is a sequence "1100000010" with A0 = 1. But if I use a single 7812 is it correct to set A0 to 1? Answer 1. The control register is a 10 bit register and the bits driven into the part on the first 10 falling edges of SCLK following TFS going low are the bits written to the control register. However, the control register itself in not updated until the 13th rising edge of SCLK (Point B on Figure 18 in the datasheet) so 13 SCLKs are needed for a full update of the control register. In our "quick evaluation seup" on page 13 of the datasheet we use 16 bits as this is a common standard for SPI communication. If 16 SCLKs are applied the unused SCLKs are just ignored. 2. A0 is essentially the address of the part. It is both a pin on the part that must be hardwired either high or low and one of the bits in the control register. It allows two devices to share the same serial bus. If the external pin is hardwired high - you must write "1" to the A0 bit in the control register to communicate with the device. If the external pin is hardwired low - you must write "0" to the A0 bit in the control register to communicate with the device. Only the serial port of the device that received the last valid control byte will attempt to drive the serial bus on the next serial read. In your case when you are using a single AD7812, you may leave A0 as "0" as long as the external pin is hardwired low.

Power-On-Reset problem on the AD7812

Question After power-up it may happen - the exact circumstances are not quite clear - that the AD7812 enters some indefinite state. Starting an conversion and reading out the result of the previous conversion produces false values.It seems the data shift register was initialy loaded with a certain value, and this value is shifted out every time a new conversion is started - no matter what channel is chosen. This can only be explained with a complete disturbance of the conversion- and write-to-shift-register- function. We tried to get the AD7812 out of this state by writing all possible patterns to the AD-control register. We found out that we can enter a correct state of operation if we set PD0 and PD1 to '0' (once during a so called power-up-routine) before starting normal write-read-operation. We can definitly say there is no input (digital/analog) which is above VCC even during power-up. VCC is rising from 0 to +5V within about 40 ms. Do you know this problem? Is our 'selfmade' power-up procedure a legal means to correct our problem ? Answer There is a known issue with the power on reset on the AD7812. Below is more complete description of the problem: If Vdd drops down to the region 0.8-1.2V and is then brought back up again, the part is not able to reset itself properly and the device will lock up and only output the last converted code. The solution is to ensure that Vdd drops all the way to 0V when the power supply is cycled. There are number of ways in which this can be achieved. The simplest is to add a resistor across the power supplies but this obviously increases the power supply of the circuit as a whole. A FET current source across the power supplies can also be used to discharge the large capacitors which are used for power supply decoupling. The AD7812 is currently (7/6/2000) being re-designed to resolve this problem. There is no confirmed schedule for release of the re-designed part. At the moment I can confirm that cycling the Power Down bits P1 and P0 will resolve this problem. -------------------------[07/07/2000] - Donal Geraghty Yes the customer has found our Power-On-Reset problem. The good news is that on the 7811/12 , you can get around it in software in exactly the manner he describes. Exercising the power down bits to effectively power-down the part AFTER Vdd ramp-up resets the internal oscillator which controls conversion. Once reset the oscillator is fine and the part will work as normal. FYI On the other members of this family , the 7810/23 and 7813/19, you can’t power-down the parts in software. Power-down happens after a conversion is complete depending on the status of CONVST. But with the Power-On-Reset problem a conversion may never happen because the oscillator ‘locks-up’. Hence the only way out is to power down and up the part.

AD7812 power-up problem

Question We are using AD7812 and during the tests the ADC stopped working and it returns the same values on all channels (for example 1.58VDC). We need to turn off the equipment to recover the ADC. We also tried to shut down only the regulator (the equipment is kept turned on) that supplies voltage to the ADC, but the ADC doesn't turn off completely...it has always 2.2VDC on its VCC pin. What could be the reason behind? As we need to turn off the ADC when it stops working, could we use another ADC(with the same pinout and footprint) that have a “reset pin”? Answer There have been cases in the past that the AD7812 does not power up correctly if power is removed from the device and the supply voltage doesn’t go all the way to ground. This is what you could be seeing here. In order to fully reset the device, you should write to the control register and set the power down bits (PD0, PD1) to zero followed by a write to the control register setting the power down bits to 1. This action of cycling the power bits has the effect of resetting the AD7812 and should return the part to normal operation under all conditions. We recommend in the datasheet that customers do this software power cycle in all power up cases. This recommendation is included in the datasheet on page 12 top right corner. I expect you are seeing 2.2V on the VCC pin because another pin is pulling it up via the internal ESD diodes, for example if there is a voltage on the analog inputs. Regarding your request for another ADC recommendation, I hope the reset procedure above will solve the problem. However, we can say that the AD7812 is a very old part and I would recommend newer designs from ADI since they will prove cheaper and possibly easier to use. A good alternative to the AD7812 would be the AD7939.

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AD7812 Production
The AD7811 and AD7812 are high speed, low power, 10-bit A/D converters that operate from a single 2.7 V to 5.5 V supply. The devices contain a 2.3 Âµs...
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AD7812 on Analog.com