Oznog
Active Member
I have been trying to interface 8 thermocouples, which generates low imedance signal around 20 mV, to a PIC's ADC stage, with high accuracy. There are two thermocouple types involved, and each requires a different gain. The signal is far to small to measure in itself, it's also so small that the offset of most opamps create too much error. So I settled on an LTC1053 dual chopper opamp, which has an extremely low offset. They signals are switched through a CD4051 analog mux.
The problem arose when I found out that one of the thermocouple styles used doesn't even isolate the junction from ground! This is a problem! The negative may or may not be above ground. At worst, it could be 20mV below ground.
I need a minimum of components here. This is difficult. A proper differential stage requires a lot more amps, for one. I could do a single opamp differential amp, but I'm not sure if the low input impedance will be a problem (the analog switch increases the signal's impedance into the amps), and I'm a bit concerned about the resistor matching. And I need a negative supply generator (MAX1044). I would need a pair of CD4052. The PIC pins require protection (probably just a resistor) for the case where the amp could put out -5V to the ADC input, likely due to a disconnected lead. The small box I had just got a lot larger.
The only good solution I could come up with was to use a LTC2418 ADC, which solves all these problems- with a 24 bit output, resolution is 300nV without gain, has 8 differential inputs, and has a fairly unique capability of digitizing signals up to 300mV below ground with no negative voltage supply.
It seems like a miracle, but problem is, this is a 28 pin SSOP pkg (not just SMT, but the closest SMT pins around). I want to make a bunch of these, but this is a terrible problem to solder. I'm having a protoype board made to see how hard it's going to be anyways.
Any advice? Ideas? Does anyone know of another ADC capable of doing the job within a DIP pkg, or at least a SOP?
The problem arose when I found out that one of the thermocouple styles used doesn't even isolate the junction from ground! This is a problem! The negative may or may not be above ground. At worst, it could be 20mV below ground.
I need a minimum of components here. This is difficult. A proper differential stage requires a lot more amps, for one. I could do a single opamp differential amp, but I'm not sure if the low input impedance will be a problem (the analog switch increases the signal's impedance into the amps), and I'm a bit concerned about the resistor matching. And I need a negative supply generator (MAX1044). I would need a pair of CD4052. The PIC pins require protection (probably just a resistor) for the case where the amp could put out -5V to the ADC input, likely due to a disconnected lead. The small box I had just got a lot larger.
The only good solution I could come up with was to use a LTC2418 ADC, which solves all these problems- with a 24 bit output, resolution is 300nV without gain, has 8 differential inputs, and has a fairly unique capability of digitizing signals up to 300mV below ground with no negative voltage supply.
It seems like a miracle, but problem is, this is a 28 pin SSOP pkg (not just SMT, but the closest SMT pins around). I want to make a bunch of these, but this is a terrible problem to solder. I'm having a protoype board made to see how hard it's going to be anyways.
Any advice? Ideas? Does anyone know of another ADC capable of doing the job within a DIP pkg, or at least a SOP?
