The cookies we use can be categorized as follows: Strictly Necessary Cookies: These are cookies that are required for the operation of or specific functionality offered. Finally, because the DAC + 1Meg are essentially functioning like a current source rather than a voltage source, this circuit is not sensitive to output impedance of the DAC itself. The second 1Meg resistor simply ensures that there is zero offset adjust when the DAC output is at mid-scale. That’s because the common-mode level shift feature of the differential op amp ensures that the op amp input nodes are biased well above ground even when the inputs to the resistors are at ground. And it is okay for the DAC to only swing positive even if node “A” is set at ground and the input signal swings above and below ground. The resistor ratio of 1Meg/1k = 1000 divides the step size of the DAC, so that now you can make adjustments of 1.25µV using a 12-bit DAC. Rather than driving node “A” directly, the DAC connects through a high-value resistor directly into the summing node of the op amp. & amp lt img src='' alt='Offset Adjust for a Differential Op Amp Driving an ADC'& amp gt įigure 2 solves all of the problems mentioned above. Unfortunately, even buffered V OUT DACs such as LTC2630-12 have impedance on the order of 100Ohm at 100kHz, which would cause errors in the circuit operation.
Finally, if the input signal changes relatively quickly, then node “A” needs to remain low impedance at these higher frequencies. That means the DAC needs a negative supply, which is not always available and in any case is not always supported by modern DACs. Second, if the input signal swings symmetrical around ground (sometimes called “bipolar”), then the DAC needs to make adjustments above and below ground. For example, a 12-bit DAC on a 5V reference results in adjustment steps of 1.25mV, which can often be too coarse. First, you need a quite high resolution DAC to make fine adjustments. While this arrangement works in theory, there can be some practical problems. So if you make small adjustments in the DC voltage at node “A”, then you are indeed adjusting the offset of the system.
Node “A” is the node with respect to which the input signal is measured. You can do this using a DAC such as the LTC2630-12, but where do you connect it into the circuit? While both of these parts already have quite good DC accuracy in terms of offset voltage, sometimes you want to further tweak the offset of the circuit. For example, the LTC6362 is a low power differential op amp that is a good choice to drive 16- and 18-bit SAR ADCs such as the LTC2379-18 family. Offset Adjust for a Differential Op Amp Driving an ADCįully differential op amps are useful components to interface a single-ended signal to a differential-input ADC.