Agilent Technologies 35670-90066 manual A2 Input, Circuit Descriptions

Circuit Descriptions	Agilent 35670A
A2 Input

	A2 Input
	The A2 Input assembly is the input assembly for the four channel analyzer. The four
	channel analyzer contains two A2 Input assemblies. The A2 Input assembly
	connected to J1 on the Motherboard conditions the channel 1 and channel 3 input
	signals before they are sent to the analog-to-digital converter on the A5 Analog
	assembly. The A2 Input assembly connected to J2 on the Motherboard conditions the
	channel 2 and channel 4 input signals before they are sent to the analog-to-digital
	converter on the A5 Analog assembly. The Input assembly sets the voltage ranges,
	conditions the input signals, and filters out alias components. Signal conditioning is
	done with relays, high and low buffers, and a series of amplifiers and attenuators. In
	addition, autozero DACs compensate for any dc offset added to the signals by the
	circuits on this assembly. The assembly also monitors the input signals for common
	mode or differential overloads and for half-range conditions. Unless stated otherwise,
	the following description applies to the block diagrams for all four channels.
ICP Source	Supplies power to transducers such as accelerometers when enabled. The ICP Source is a 4.25
	1.5 mA floating current source. The power for the current source is obtained by switching the
	15 V supplies at the sample frequency (262 kHz), isolating through a 1:1 transformer,
	rectifying and filtering. When disabled, the source is disconnected by a relay and the switching
	power supply is disabled.
Input Relays	Select one of the 0 dB, 20 dB, or 40 dB pads and configure the input. The relays are controlled
	by Relay Select & Energize.
Relay Select &	Selects and energizes the input relays. Relay Select & Energize is controlled by the IIC
Energize	Interface.
High Buffer	Buffers the HIGH input signal (BNC center conductor). The High Buffer contains a bootstrap
	circuit that prevents harmonic distortion and large common mode signals from saturating the
	buffer.
Low Buffer	Buffers the LOW input signal (BNC shell). The operation of the Low Buffer is identical to the
	High Buffer.
+10 dB Differential	Begins the gain and attenuation stages. This amplifier subtracts the HIGH and LOW input
Amplifier	signals from the high and low buffers. The common mode rejection adjustment, adjusts the
	+10 dB Differential Amplifier to reject common mode signals.
0 /–12 dB Amplifier	Can attenuate the signal by 12 dB.
0 /+14 dB Amplifier	Can amplify the signal by 14 dB.
Step Attenuator	Can attenuate the input signal from 0 dB to –14 dB, in 2 dB steps.
+2 dB Amplifier	Adds a gain of 2 dB and allows the DC Offset DAC to vary the dc offset of the input signal.
	The purpose of the amplifiers and attenuators up to this point is to ensure that the input signal
	does not overdrive the anti-alias filter or analog-to-digital converter.

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