ADC for pH and conductivity sensor

How many bits does a pH or conductivity sensor have? I need a Analog-to-Digital-Converter for these sensors. Thus I need to know how many bits are required in order to purchase the right ADC. I am not sure if 8, 12 or 16 Bit. Can anyone help me out here?

EDIT: For now we have HACH sensors connected to the SC100 controller, but I am searching other sensor which do not require a controller. Yes 4-20 mA is the output of the sensors I am looking for as well as the controller from HACH SC100.

I don't know the voltage ouput yet.

I want to connect the ADC to a Simatic S7-1200.

You have not specified which sensors exactly you're referencing. So I'm assuming the pH or conductivity sensor is a bare analogue sensor and not an active device.

Bit resolution is a symptom of quantising a continuous quantity. In other words, the analogue signal from your sensor has no resolution (or, if you will, can be assumed to have an infinite resolution). It's only when you try to digitise the continuous quantity that the bit resolution is defined.

The number of bits you choose 8, 12 or 16 is dependant only on the resolution you require on the digital side. But you may need to take into account other factors such as the reference voltage you will use to power the sensor.

A pH sensor outputs an analogue voltage, so you can't measure it in bits.

The correct ADC depends on what resolution you want.
Say you want a resolution of 0.02pH, with a range from 0-14pH. Then you need 14/0.02 = 700 discrete steps, which is log2(700) = ~9.45 bits. So a 10 bit ADC would cover this, giving you a resolution of 14/1024 = 0.0137 pH.
How precise or accurate the readings actually are would depend on various factors, so it may be pointless to go for more bits unless you can actually make use of the extra resolution.

EDIT - If it's just a passive sensor, you can't connect the sensor directly to the ADC, as the output impedance will be very high and the ADCs input impedance will be low in comparison, so it would load and attenuate the signal too much. Also you need some gain to match the sensor output voltage range with the range of your ADC.
What you need to do is use an opamp with a very high input impedance (e.g. FET input type) to buffer and amplify the sensor signal before it goes into the ADC.
However, the fact you mention 4-20mA output makes me think your sensor may have an opamp on board already, in which case you may be able to simply connect directly to your ADC.
To confirm this can you provide the sensor type and model number if possible, and the voltage range on it's output (plus any other info you have about it)
Also, how you are planning to send the ADC data to the computer (USB? Serial/Parallel?)

• Ok I understand. How do I exactly connect the sensor with the ADC? The sensor comes out with 4-20 mA. Which connection type should i rely on in order to get the signal into my computer? Oct 31, 2011 at 7:26
• @HarveyDent - See edited answer. Can you update your question with the requested info please. Oct 31, 2011 at 8:07