Customers who wish to evaluate the ADC through differential inputs will find themselves needing a low distortion, low noise differential sinusoidal source. Single-ended sinusoidal sources can be obtained by creating a simple Wien bridge oscillator or by using several off-the-shelf audio oscillators. Different sinusoidal sources are not common. The LTC6363 precision low-power rail-to-rail output differential op amp converts a single-ended sinusoidal source to a differential output sinusoidal source while maintaining a very high level of performance.
The circuit in Figure 1 is used with the sinusoidal sources of the DC1858A and DC1925A ADC demo boards to demonstrate circuit performance. The DC1858A is a low noise, low distortion 2 kHz sine wave generator. The typical DC1858A has a THD of -118dB and a typical signal-to-noise ratio of 104dB. The DC1925A is a demonstration board for the LTC2378-20 20-bit, 1Msps ADC with fully differential inputs. The ADC has a typical THD of -125dB and a typical SNR of 104dBFS.
The circuit of Figure 1 was constructed using the DC2319A (the demo board of the LTC6363). The part number in the schematic corresponds to the part number used by the DC2319A. Looking at the circuit of Figure 1, the sinusoidal output of DC1858A is applied to the J2 input. The V1 / 2 of the DC1925A is applied to the J1 input (1 pin of JP4). This appropriately sets the input and output common mode levels. Resistor R5 is used to match the output impedance of DC1858A, and capacitor CX filters out the noise applied by J1. The V + (+ 8V) and V - ( - 3.6V) provided by the DC1925A power the circuit. The circuit gain is determined by the ratio of the feedback resistor (R3 = R4) to the input resistance (R1 = R2). The J4 and J3 outputs are connected to the J2 and J4 inputs of the DC1925A. The RC filters of Vout + (R10, C12) and Vout-(R9, C11) minimize the output noise of the LTC6363.
Circuit performance is shown in the PScope output of Figure 2. The SNR in dBFS is obtained by adding the absolute value of the F1 amplitude to the indicated SNR. This produces 103.26 dBFS, which is 1 dB less than the typical values ​​of DC1858A and LTC2378-20. THD is -113.11dB. This is about 5 dB worse than the typical DC1858A value, but close to the typical value of the LTC6363.
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