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LeCroy DA1855A, Operation

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DA1855A Differential Amplifier

3-16

ISSUED: July 2002

DA1855A-OM-E Rev A

In some situations, the user may wish to select amplifier settings 
optimized for the lowest noise performance with lower common 
mode range. Or, the user may require the amplifier not to change 
noise or common mode range when the 

VOLTS/DIV

 setting is 

changed. These requirements can be met by selecting the 
Manual mode in the menu box of user interface window. 

Figure 3-2. DA1855A Control Menu in Manual Mode.

When set to Manual mode, the dynamic range of the 

VOLTS/DIV

 

knob is limited to the scale factors which can be obtained without 
changing the DA1855A gain or attenuation. Thus in Manual gain 
control mode, only the scale factor of the oscilloscope will be 
changed. In this mode, dedicated menu boxes are displayed for 
amplifier Attenuation and Gain. (Refer to figure 3-2).

The available attenuation values which appear in the Value box 
will be 

/10 

or 

/1

 independent of the attenuation of any external 

probe.

With software version 8.1.0 or higher, the channel 

OFFSET

 knob 

will control the amplifier’s offset rather than the amplifier’s PVG 
buttons. 

Summary of Contents for DA1855A

Page 1: ...al Amplifier is connected to an oscilloscope not provided with a ProBus interface Note Removing the ProBus interface cable with the differential amplifier still powered up requires the DA1855A to be turned OFF and ON to access the front panel controls DYNAMIC RANGE The basic amplifier dynamic range in X1 Gain and 1 Attenuation is 0 500 V Changing the gain and or attenuation will affect both the Di...

Page 2: ...A1855A amplifier s corresponding differential input The signal at each input is attenuated by a factor of ten Gain The DA1855A amplifier gain amplification is selectable between X1 and X10 The amplified signal appears at the rear panel AMPLIFIER OUTPUT connector Gain will affect the differential mode output signal by amplifying the signal difference between the INPUT and the INPUT but will not aff...

Page 3: ...ero momentarily sets the input coupling to OFF and determines the offset necessary to set the output at 0 Volt During this process the front panel input signal to the amplifier is interrupted When the Auto Zero cycle is completed the input coupling returns to its previous state Auto Zero usually takes less than one second to complete This feature allows the user to DC balance the DA1855A simply by...

Page 4: ...oint This will safely recharge the AC coupling capacitor in less than 0 3 seconds DC and low frequencies are attenuated by the AC coupling capacitor and the input resistance With the ATTENUATOR set to 10 or set to 1 with the INPUT RESISTANCE set to 1 MΩ the low frequency cut off 3dB point is approximately 1 6 Hz When the input attenuator is set to 1 the INPUT RESISTANCE may be set to 100 M Ω and t...

Page 5: ...1 where the horizontal center line represents a power supply voltage of 5 030 V the next higher line 5 050 V and the line below the center line 5 010 V In this figure noise on a 5 030 V signal is easily displayed using 5 030 Volt offset and a vertical scale factor of 20 mV div VCOMP can be used to make precise measurements of large signals by comparing the accurately known VCOMP with the unknown s...

Page 6: ...mode voltage range also applies when using attenuating probes When the DA1855A is used with attenuating probes that feature readout the PVG display is changed to indicate the voltage at the INPUT probe tip which will bring the amplifier output to zero When connected to a LeCroy oscilloscope via the ProBus interface the oscilloscope OFFSET control increments or decrements the PVG s output voltage a...

Page 7: ...ement buttons are oscilloscope waveform related by factory default The increment buttons move a displayed oscilloscope waveform upward and the decrement buttons move the waveform downward independent of the PVG polarity Decrements from a positive voltage will roll smoothly through zero This is known as roll through zero mode Toggle PVG modes To change from roll through zero to absolute mode of ope...

Page 8: ...ators LEDs across the top of the DA1855A front panel show the total gain from the instrument input to output Logic within the amplifier includes the gain internal attenuation and probe attenuation factors when readout encoded probes are used to determine the effective gain When the X1 light is ON the overall amplifier voltage gain amplification is unity Similarly X10 indicates an overall amplifica...

Page 9: ...cating the amplifier is reset When the ATTENUATOR is set to 1 an input signal of approximately 19 Volt will activate the overload protection circuit Fast transients will draw up to about 70 mA of input current for a brief period before the input coupling relay acts to disconnect the input Caution Inputs in excess of 250 Volt may cause permanent dam age to the DA1855A The input is not disconnected ...

Page 10: ...ted by the input attenuator or probes whereas the input signal is Therefore the effective range of VCOMP is increased by a factor of 10 when the 10 ATTENUATOR is selected or a 10 attenuating probe is used to attenuate the input signal The PVG numerical display reflects the attenuator setting and probe attenuation when the probe is readout encoded As an example if there are no probes attached the 1...

Page 11: ...internalVDIFF amplifier Tables 3 3 and 3 4 will help the operator stay within the maximum input voltage limits and understand the relationship between the actual voltage applied and the effective voltage Effective voltage is always referred to the input of the DA1855A or the probe tip if a probe is used When using probes the maximum effective voltage range may be limited by the maximum voltage rat...

Page 12: ... having a 50 Ω input resistance The amplifier s output impedance is 50 Ω Without the 50 Ω load the amplifier gain will be uncalibrated and will be approximately twice the amount indicated on the front panel Proper operation of the 1 MHz or 20 MHz bandwidth limit filters requires an output load impedance of 50 Ω Remote Operation A REMOTE connector on the rear panel of the DA1855A allows total contr...

Page 13: ...A output is intended to connect directly to the input of an oscilloscope or other instrument but it is important to observe some rules so that the DA1855A delivers its specified performance CAUTION A properly terminated differential amplifier can deliver an output voltage of 0 5 Volt The output is DC coupled and will follow any DC component applied to the input Some instruments such as spectrum an...

Page 14: ...g from overload When used with instruments lacking ProBus interface the instrument s gain and position controls should be properly set to avoid displaying the non linear portion of the DA1855A s output signal when it is in overdrive This can be accomplished by observing the two following rules 1 Turn the oscilloscope input coupling to OFF or GND set the oscilloscope position control to center scre...

Page 15: ... of resolution to obtain 1mV or 2 mV div sensitivity The loss of resolution can be avoided by using this technique Any oscilloscope bandwidth limit setting may be used so long as the unlimited signal does not exceed full screen before invoking bandwidth limit Gain Control Modes When the DA1855A is connected to a LeCroy oscilloscope equipped with ProBus interface the displayed scale factor and meas...

Page 16: ...ol Menu in Manual Mode When set to Manual mode the dynamic range of the VOLTS DIV knob is limited to the scale factors which can be obtained without changing the DA1855A gain or attenuation Thus in Manual gain control mode only the scale factor of the oscilloscope will be changed In this mode dedicated menu boxes are displayed for amplifier Attenuation and Gain Refer to figure 3 2 The available at...

Page 17: ...t in very poor CMRR performance Nominally matching 1 probes however will provide excellent common mode rejection and are recommended For applications which do not require additional attenuation 1 probes present relative high capacitive loading to the circuit under test limiting their usefulness to low frequency measurements When making differential measurements accurate probe compensation is much ...

Page 18: ...ee methods of connecting probe grounds In most cases when the common mode portion of the signal consists mainly of low frequencies 1 MHz and below the probe ground leads should not be connected to the ground of the circuit under test They should be connected to each other This minimizes the effects of ground loop currents The signal corruption caused by not having the probes connected to the groun...

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