A Differential Op-Amp Circuit Collection - Parte III
4 Driving Differential Input Data Converters
Most high-resolution, high-accuracy data converters utilize differential inputs instead of singleended inputs. There are a number of strategies for driving these converters from single-ended inputs.
In Figure 14, one amplifier is used in a noninverting configuration to drive a transformer primary. The secondary of the transformer is center tapped to provide a common-mode connection point for the A/D converter Vref output.
Gain can be added to the secondary side of the transformer. In Figure 15, two single-ended op amps have been configured as inverting gain stages to drive the A/D Inputs. The non-inverting input inputs are connected to the transformer center tap and A/D Vref output.
Figure 16 shows how single-ended amplifiers can be used as noninverting buffers to drive the input of an A/D. The advantage of this technique is that the unity gain buffers have exact gains, so the system will be balanced.
Transformer interfacing methods all have one major disadvantage:
If the response of the system must include dc, even for calibration purposes, a transformer is a serious limitation. A transformer is not strictly necessary. Two single-ended amplifiers can be used to drive an A/D converter without a transformer:
Although all of the methods can be employed, the most preferable method is the use a fully differential op-amp:
A designer should be aware of the characteristics of the reference output from the A/D converter. It may have limited drive capability, and / or have relatively high output impedance. A high-output impedance means that the common mode signal is susceptible to noise pickup. In these cases, it may be wise to filter and/or buffer the A/D reference output:
Some A/D converters have two reference outputs instead of one. When this is the case, the designer must sum these outputs together to create a single signal as shown in Figure 20: