Leading Features

• DC to 100 MHz Bandwidth

• 100000 to 1 CMRR

• Gain 1 or 10

• 100 ns Overdrive Recovery

• Very Low Noise

The DA1850/1855 is a stand-alone, high-performance 100 MHz differential amplifier. It is intended to act as a signal conditioning preamplifier for oscilloscopes, digitizers and spectrum analyzers, providing differential measurement capability to instruments having only a single-ended input. When used with a DA1850/1855, most good-quality oscilloscopes can obtain Common Mode Rejection Ratio (CMRR) and overdrive recovery performance levels previously unobtainable.

Amplifier gain can be set to 1 or 10. A built-in input attenuator can be separately set to attenuate signals by a factor of 10, providing gains of 10, 1, or 0.1 and common mode dynamic range of ±15.5 V (÷1) or ±155 V (÷10). Optional probes increase the maximum input signal and common mode ranges in proportion to their attenuation ratio but do not exceed their maximum input voltage rating. Effective gain of the DA1855, including probe attenuation, amplifier gain and attenuator settings, is automatically displayed.

The DA1855 has a bandwidth of 100 MHz, but any one of the three 3-pole bandwidth limit filters can be selected to reduce bandwidth to
10 MHz, 1 MHz or 100 kHz to limit noise above the frequency of interest.

The DA1850/1855 output is carefully limited to ±500 mV, so that the oscilloscope is not overdriven by large inputs. This allows many oscilloscopes to directly measure the settling of D/A converters with 14-bit (60 ppm) precision, better than any other differential comparator.

The DA1855 features a built-in Precision Voltage Generator (PVG) that can be set to any voltage between ±15.5 volts (±10 volts in Differential Offset) with 100 µV resolution. Each digit of the voltage generator output can be individually incremented or decremented and the sign changed between + and -. The PVG's output can be selected as an input to the inverting (-) input of the amplifier for operation as a differential comparator or applied internally as a true differential offset voltage. The PVG is also available for external use through a rear panel connector.

The DA1850/1855 operates from 100 to 250 VAC line without line switching.

High-performance differential probes such as the DXC100 10X/ 100X high CMRR probes are recommended.

Overdrive Recovery

The DA1850/1855, by limiting the signal applied to the input of an oscilloscope, will greatly improve the oscilloscope's effective overdrive recovery. For example, an oscilloscope with very good overdrive recovery will tolerate input signals of about ±2 V when its attenuator is in the straight through position (input signal not attenuated). The scope will recover within 0.2% (2000 ppm) in 50 ns, producing an error signal of 4 mV. Such an error signal is better than most oscilloscopes specify.

Using the same oscilloscope and the DA1850/1855 set at x10 gain, the oscilloscope error will be reduced to about 1 mV by the amplifier's limiting. At the same time, the amplifier increases the gain by 10 resulting in an equivalent error of 0.1 mV referred to the DA1850/1855 input 100 ppm of the 2 V input signal.

Unlike the oscilloscope, the DA1850/1855 handles input signals up to 15.5 V with no increase in error. For a 10 V input signal, the same
0.1 mV error represents just 10 ppm of the signal. This improves on the raw oscilloscope performance by a factor of about 200.

Comparator Mode

The DA1855 becomes a differential comparator when the internal Precision Voltage Generator (PVG) output is selected as the amplifier's inverting (-) input. In this mode, the DA1855 can be used to very accurately measure relatively small signals that are riding on large AC or DC components. Due to the precision of the voltage generator, an oscilloscope, when used with the DA1855, can make voltage measurements that are much more accurate than the oscilloscope is capable of by itself. The output of the PVG is available for external use via a rear panel connector.

The DA1850 also functions as a differential comparator when an optional voltage source is supplied.

True Differential Offset Mode

The DA1855's built-in Precision Voltage Generator can be used to generate a true differential offset while still allowing both inputs to be used as differential inputs. The offset range can be as high as ±50,000 divisions and the generator has 5 ¹/² digit resolution. This mode facilitates making measurements such as changes to a transistor's base to emitter voltage caused by variations in temperature and/or current. Used in this mode, the voltage generator can be set to a value that will zero out the static value of the junction's on voltage. The DA1855's differential measurement capability will reject any dynamic signal common to both sides of the junction, and the oscilloscope is left to measure only the changes in the junction voltage.

Precision Voltage Generator (DA1855 only)

The Precision Voltage Generator is controlled from the DA1855 front panel. Each digit can be individually incremented or decremented and rollover automatically carries to the next digit. The display is treated as an absolute number, and the sign can be changed without altering the value. The decimal point placement is calculated to take into account the DA1855 mode (Comparator or Differential Offset), gain, and attenuator settings, along with any probe attenuation. Thus the display accurately reflects the proper effective value of the Comparison or Differential Offset Voltage. The range of the generator is ±15.5 V for the comparator mode and ±10.0 V for differential offset mode. The generator's temperature-controlled oven allows for a DC accuracy specification of 0.05% +500 µV.

The DA1850/1855 input is protected to ±250 V, and will automatically disconnect large signals and will require a manual reset.

Autobalance

Each time either gain setting button is pressed, the DA1850/1855 automatically adjusts the amplifier's DC Balance.

Specifications

Model	Description
DA1855A	100 MHz Differential Amplifier with Precision Source; Probe Sensing
DA1855A-PR2	2 Ch, 100 MHz Differential Amplifier and Comparator with Precision Voltage Source and Bandwidth Limit (10 MHz, 1 MHz, 100 MHz).
DA1850A DISCONTINUED	100 MHz Differential Amplifier
DA1850A-PR2 DISCONTINUED	2 Ch, 100 MHz Differential Amplifier
Model	Description
DXC100A	100:1 or 10:1 Selectable 250 MHz Passive Differential Probe Pair
DXC200	1:1, 50 MHz Passive Differential Probe Pair
DXC350A DISCONTINUED	100:1, 250 MHz, 92 MOhm, 2.6 pF Differential Probe Pair
DXC5100	100:1, 250 MHz, 2.5kV High Voltage Probe Pair
DA101	10:1, 1 MOhm Passive Attenuator
DA18XX-RM01	Rackmount Adapter for DA18XX-PR2, Size (2U) (Must be ordered with instrument)
To view a list of DISCONTINUED PRODUCTS click this link.