The Tektronix ADA400A Differential Preamplifier allows direct oscilloscope measurements of very low amplitude voltages and signals which are not ground referenced. The high impedance of both inputs eliminates the need to add additional ground points in the DUT, thereby avoiding circulating currents which disturb the measurement or the circuit. The high gain and extremely high common mode rejection provide usable measurements of voltages as low as 5 V, even in high noise environments. Selectable bandwidth limiting reduces normal mode noise from digital logic, switch mode power supplies and line frequency sources. Adjustable differential offset allows the user to null out transducer bridge bias and galvanic potential from the test setup. An "infinite impedance" mode can be selected in the two highest gain ranges allowing accurate measurements of signals with high source impedance.