ADS1220: Designing with ADS1220IRVAT

Shibin,

One simple approach is to use an instrumentation amplifier (INA) to convert the bridge output from a differential signal to a single ended signal.  The output of the INA is then divided by a voltage divider and connected to the ADC input.  One way to use the INA is to set the gain to 1 and the offset so that the output of the INA is near 2.5V.  2.5V is the best common mode input for the ADS1220 when it has a 5V supply.  The voltage divider at the output of the INA will keep the ADC input in a safe range even if the INA output rails.  The ADS1220 PGA can then be used to gain up the INA / bridge output.  The divider in the example circuit below takes the INA full scale output of 12V and translates it to 5V.  The INA offset is set to 6V so that the divider reduces it to 2.5V which is the ADS1220's optimal common mode input.  The INA uses a 12V supply which optimizes the common mode input of the INA to 6V (i.e. it matches the bridge common mode range).  If you need additional gain, then we will need to understand your bridge characteristics better.  You could use a DIP Adaptor EVM to test the INA with the ADS1220EVM.  There are many different INA that will work for this application.  I suggest the INA849 as it has low noise and good offset, but any INA with a supply range greater than 12V should work.  Some things to consider are noise, offset voltage and offset drift.  I hope this helps.

Art

Shibin,

1. The easiest and best approach is to set the bridge excitation to 5V an directly drive the input of the ADC.  Bridge sensors are often characterized and specified at a particular excitation voltage, but should work at lower voltages.  The exception to this is when the bridge includes active devices (e.g. an amplifier).  If your bridge is just a resistive bridge I would strongly suggest that you investigate if you could use a 5V excitation voltage.  If you can, you will just use the internal PGA to amplify the differential bridge output. 

2.  Regarding the INA, You need to consider the difference between common mode voltage and differential voltage.  The 6V output of the bridge is a common mode signal.  If both of the outputs of the bridge are at 6V then the differential voltage is 0V.  The INA will reject the common mode 6V and amplify the differential signal.

3.  You are correct that the bridge output will be a differential 24mV with a 12V excitation.  Also, for A 12V excitation the common mode output is 6V.  The differential signal will be amplified by the INA and the common mode signal will be rejected.  If the reference pin is set to 6V then the output of the INA will range from 6V to 6.024V. The divider will then translate this to 2.5V to 2.51V.  You can configure the ADS1220 to accept a differential voltage and apply the INA output to the positive input and a fixed 2.5V to the negative input.  This will reject the 2.5V common mode from the INA and the differential will be 10mV.  You can then use the ADS1220 internal PGA to amplify the differential signal.  One problem with this approach is that all the different components will have some tolerance so the 2.5V output from the INA and the 2.5V reference in the negative input will have errors and will not cancel exactly.  Thus there will be an offset introduced by this difference which will limit the gain range.

4.  Alternatively, you can use gain in the INA.  If the sensor outputs 0V differential when it is balanced (i.e. no stimulus), and 24mV under full load than you can choose a gain and offset to output a single  ended signal that the ADS1220 can use.  For example, you can use a gain of 300 and an offset of 1V for the INA.  This will make the INA output range from 1V to 8.23V.  The output of the voltage divider is then 0.417V to 3.429V.  This signal could be applied to a single ended ADC input.  Keep in mind that bridge sensors typically have offset and gain error so the balanced output will not be exactly zero and the full scale output will not be exactly 24mV.  Thus you need to leave range below zero and above the full scale to account for the gain and offset errors.  That is why the output is scaled from 0.417V to 3.429V instead of 0V to 5V.  You may want to add more margin if your sensor has large gain and offset error specification.  Attached is a TINA spice file that shows this circuit.  This circuit will require that you generate a 1V offset (see voltage divider and op amp in schematic below).  Again, keep in mind that all this extra complexity can be eliminated if you use the bridge with a 5V supply.  Below are pictures of the balanced output and the full scale output.

5. Please see this document on Bridges.  It is very detailed and may be helpful to you:  A guide to bridge measurements