ARD-LTC2499 Hardware Reference
Hardware Configuration
In order to adapt to a wide variety of use cases and ArduinoTM configurations, the ARD-LTC2499 board has a number of configuration jumpers.
- JP1A & JP1B - I2C Pull-up Enable
- JP2 - I2C Pin Selection
- JP3 - Onboard 5V Linear Regulator Enable
- JP4, JP5 & JP6 - LTC2499 I2C Address
- JP7 & JP8 - EEPROM I2C Address
JP1A & JP1B - I2C Pull-up Enable
The I2C bus depends upon a set of pull-up resistors to function correctly. Each I2C bus should have one and only one set of pull-ups enabled. The ARD-LTC2499 includes a set of 1.6k pull-up resistors that can be enabled by jumpering JP1A and JP1B. Note that Arduino boards do not provide I2C pull-ups, so at least one board connected to I2C must provide them.
JP2 - I2C Pin Selection
Not all Arduinos provide I2C on the same pins. With the Rev 3 hardware designs, there are now a dedicated set of pins above AREF and GND marked "SDA" and "SCL". If you see those on your board, then place jumpers between pins 3-5 and 4-6 on JP2 to connect the I2C bus to them.
If you board does not have explicit SDA and SCL pins, then most likely they're shared with pins A4 and A5. Place jumpers between pins 1-3 and 2-4 to select those instead.
JP3 - Onboard 5V Linear Regulator Enable
To free the LTC2499 from any noise on the Arduino's 5V supply caused by the AVR or other shields, the ARD-LTC2499 includes an onboard 78L05 regulator coming from the Arduino's "VIN" pin. If you are powering the Arduino with >8VDC via the power jack, you can use the ARD-LTC2499 onboard 5V regulator to provide nice quiet power by jumpering pins 2-3 of JP3.
If you are powering the Arduino off of the 5V provided by USB or via some other means that does not place >8VDC on the VIN pin, then the regulator will not work and must not be used. JP3 must be placed between pins 1-2 in this case.
JP4, JP5 & JP6 - LTC2499 I2C Address
Each device on the I2C bus must have a unique device address. The LTC2499 has 27 possible addresses depending on how JP4, JP5, and JP6 are set. Each may be set high (jumpering pins 1-2, the two closest to the LTC2499), low (jumpering pins 2-3, the ones furthest from the 2499) or floating (no jumpered pins). Please refer to the Table 5 in the LTC2499 datasheet or the Ard2499 library header for corresponding address values. JP4 corresponds to CA0, JP5 to CA1, and JP6 to CA2.
JP7 & JP8 - EEPROM I2C Address
Each device on the I2C bus must have a unique device address. The 24AA025E48 EEPROM that provides both non-volatile storage and the globally unique ID number has four possible addresses depending on how JP7 and JP8 are set. Each may be set high (no jumper) or low (jumper in place). Please refer to the Figure 5-2 in the 24AA025E48 datasheet (we use the SOT-23 version, so A2 is always '0' - otherwise JP8 is A1 and JP7 is A0) or the Ard2499 library header for corresponding address values.
Usage
The useful measuring range of the inputs is 0V to 2.048V in single-ended mode, and a +/-2.048V in differential mode. Be careful of common mode limits, however. Inputs have an absolute maximum common mode limit of -0.3V to +0.3V above VCC (nominally 5V) before damage to the part will occur, so while the differential inputs can have -2.048V between them, neither side of the input may violate the limit of -0.3V to 5.3V without damaging the part.
To eliminate any possibility of introducing gain or offset errors from buffering, the pins of the LTC2499 are brought directly to the terminal headers. Thus, please be careful when handling and using the board. Errant electrostatic discharge or voltages applied outside the absolute maximum limits specified in the datasheet can cause damage rather quickly.
The 4.096V Reference output on J3 is straight off of the LT6654 reference output, with no buffering. The LT6654 is rated for a maximum of +/- 10mA. Also note that any current drawn from this pin will degrade the accuracy of the reference by a small amount (see the Electrical Characteristics table, "Load Regulation").