The REF34xx-EP is a new voltage reference from TI that offers an output voltage accuracy of ±0.05%.
There are two devices within this series: the REF3425-EP for generating Vref=2.5 V, and the REF3440-EP for Vref=4.096 V. The REF34xx-EP provides a maximum output voltage temperature coefficient of 6 ppm/°C over the entire operating temperature range, which extends from -55°C to 125°C. The maximum quiescent current of the device is 95 μA in active mode and 3 μA in shutdown mode.
The typical device dropout voltage (VDO) is 50 mV, and a wide input voltage ranging from Vref+VDO to 12 V is supported. The output 1/f noise of the device is 3.8 μVp-p/V from 0.1 Hz to 10 Hz. Some common applications of this device are battery-operated equipment, industrial instrumentation, power monitoring, and field transmitters.
Let’s take a closer look at this new voltage reference.
Functional Block Diagram
The functional block diagram of the REF34xx-EP is shown in Figure 1. As you can see, a bandgap reference is the core of the device.
The Enable Pin
Pulling the enable pin (EN) below 0.5 V places the device in a low-power mode where the quiescent current is less than 3 μA. Applying a voltage higher than 1.6 V will make the chip exit the low-power mode and become operational. Any voltage between 0.5 V and 1.6 V can increase the supply current and even prevent the device from functioning correctly. Hence, the signal applied to the EN pin should have sufficiently fast rising and falling edges.
The specified value for the input voltage is from Vref + VDO to 12 V. The power supply should have a fast rising edge during start-up and low output impedance; otherwise, the quiescent current can momentarily increase to levels that exceed the typical values. Additionally, a slow rising or falling edge on this pin can cause transient anomalies, such as overshoot, on the device output. The datasheet recommends slew rates of about 6 V/ms for the input voltage.
Kelvin Sensing Support
We usually place the voltage reference as close as possible to its load; however, sometimes this is not convenient or possible. In these cases, there can be a relatively long PCB trace between the voltage reference and the load, and we may have to consider the IR voltage drop along the trace. For example, a 1-in long trace carrying a current of 10 mA can produce a voltage drop of about 1 mV.
In such cases, the reference IC is functioning correctly and is producing its output voltage within the specified accuracy; however, the load gets a slightly different voltage because of the IR voltage drop along the trace. To circumvent this problem, the REF34xx-EP allows us to implement the four-terminal (or Kelvin) sensing technique.
The Kelvin sensing technique uses a pair of wires (or PCB traces) to carry current to the load and another pair of wires to sense the voltage drop across the load. The chip terminals connected to the voltage-sensing lines are designed to have high impedance. Hence, only a small current flows through the voltage-sensing lines and the IR drop across them is negligible. In this way, the REF34xx-EP can minimize the error from trace voltage drop and accurately sense the voltage applied to the load.
The following figure shows the concept of Kelvin sensing to measure a load resistor Rsubject. Please refer to this page in the AAC textbook for more details.
The four connections for implementing the Kelvin measurement are sometimes called the force and sense connections. As you can see, there are two output terminals, OUTF and OUTS, and two ground terminals, GNDF and GNDS, in Figure 1. The F and S letters correspond to the force and sense connections, respectively.
The cost of using the Kelvin connection is routing an extra pair of traces on the PCB. When this type of measurement is not required, we can simply connect the corresponding force and sense pins together.
Solder Heat Shift
Soldering heat can shift the reference output voltage slightly. The REF34xx-EP datasheet reports the test results for 32 devices that experienced the reflow soldering profile shown in Figure 3 below.
The test results show that most of the chips have a shift of less than 0.01% in the reference output voltage (see Figure 4). However, there are chances of even higher shifts depending on the size, thickness, and material of the PCB. Besides, if the board is going to experience two reflows, it’s recommended to solder the voltage reference in the second pass of the process to minimize exposure to soldering heat.
Designing for Dual-Supply Reference
If your particular application requires both positive and negative references, you can combine the REF34xx-EP with a zero-drift amplifier such as the OPA735, as shown in Figure 5 below. This design creates both +2.5 V and -2.5 V references.
R1 and R2 must have very high precision and matched temperature coefficients in order to ensure that the negative reference is accurate.
In this article, we looked at TI’s new voltage reference, the REF34xx-EP. The device exhibits low drift, has a low dropout voltage, and draws a small amount of quiescent current. Additionally, it supports a Kelvin connection, which is helpful when we cannot achieve a very small physical separation between the voltage reference and the load.