Hi,
I'd like to know how the ADS1147 / 48 input impedance changes when the Idac is ON: in this case there is a "current surce" connected to the input pin thus the impedance should be a little bit lower; I'd like to have a numerical value.
Thanks
Gabriele
Hello Gabriele,
The IDAC currents are independent of the internal programmable gain amplifier (PGA) of the ADS1148. The IDAC programmable current source connects to the selected channel and sources the programmed current as shown on the block diagram below. The ADS1247/48 internal programmable Gain Amplifier input impedance remains high between 5000 MOhm to 300 MOhm depending on data rate as specified on table 7 (p25) ADS1248 datasheet; independent of the IDAC settings.
The IDAC currents are typically used for current excitation for RTD's or thermistors. RTD's sensors can be connected in different configurations (2-Wire, 3 wire and 4 wire). In the case where the IDAC current is programmed to flow through the same channels that are used to sense the measurement, for example on the RTD 3-wire configuration, the user has to ensure that the trace impedance connecting the the AINP and AINN inputs are closely and carefully matched, since the IDAC excitation flows through the same path. In the case of the 4-wire configuration, the IDAC current flows through an independent path and therefore the excitation path of the RTD is independent from the sensing RTD path, providing the highest level of accuracy.
Please find the SBAA180 application note below providing a detailed examples and explanation of the ADS1248/7 measuring RTDs in the Two-Wire, Three-Wire and Four -Wire RTD configuration.
http://www.ti.com/lit/an/sbaa180/sbaa180.pdf
I am not sure I follow the question, please explain which impedance are you referring to? what kind of sensor is being measured in the application.
Thank you and Best Regards,
Luis
Note: The ADS1148/7 and ADS1247/ADS1248 have a very similar PGA input stage and IDAC excitation currents.
Hello Gabriele,
- An ideal current source will have an infinite output impedance. As you have mention, a real current source will have a finite but relatively high impedance in the order of MegaOhms. A high output resistance in the current source is desirable; since this reduces the dependency of the output current to the output voltage; where the output current will change slightly as the output voltage present changes. In the typical ADS1147/8 application, the IDAC excitation current will source current to a RTD or a thermistor sensor. In the recommended configuration (shown below); the ADS1148/7 device is set up performing a ratiometric measurement, where the IDAC current is used to excite the sensor and also to produce the reference voltage across the RBIAS resistor. In this ratiometric configuration, the errors due to the absolute value of the IDAC and the IDAC current drift are eliminated, since the changes of the IDAC current are reflected both at the input path and the reference path. The RBIAS resistor generating the voltage reference must be a precision low drift resistor.
- The amplifier measuring the sensor is required to have high input impedance. The output impedance of the sensor being measured could potentially interact with the input impedance of the instrumentation amplifier and produce an attenuation or gain error. Also, a low impedance amplifier would have a loading effect in the circuit that could cause errors in the measurement. When performing measurements using the ADS1147/8, the input voltage measurement is made using the internal Programmable Gain Amplifier (PGA) of the device. The internal PGA is optimal and provides high impedance and low input bias currents.
In the 4-wire RTD configuration below, the excitation current path is independent of the sense path; and since the internal PGA has high input impedance and low input bias current; the errors due to the series resistance in the signal path are negligible.
In the 3-wire configuration, the designer has to ensure that the impedance/resistance through the AINP and AINN signal paths are closely matched, since the excitation current flows through to them. The (2) IDAC currents are closely matched on the device, however, in applications that require higher accuracy, the user could perform the procedure attached to calibrate for the IDAC current mismatch. The 3-wire configuration is also ratiometric, where the absolute value of the IDAC is not critical.
4505.IDAC_Mismatch correction.pdf
Since the output impedance of the current source relates only to the absolute value of the current source, and since the configuration below is ratiometric, the output impedance of the current source does not contribute any significant error in the RTD/sensor measurement.
Thank you and Best Regards,
Luis
